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Research article Journal of the Geological Society Published Online First https://doi.org/10.1144/jgs2019-156

The nature and origins of decametre-scale porosity in Ordovician carbonate rocks, Halahatang oilfield, Tarim Basin, China

Estibalitz Ukar1*, Vinyet Baqués1, Stephen E. Laubach1 and Randall Marrett2

1 Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, University Station Box X, Austin, TX 78713-8924, USA 2 PO Box 432, Island Park, ID 83429, USA EU, 0000-0002-1913-6843; RM, 0000-0002-7124-0680 * Correspondence: [email protected]

Abstract: At >7 km depths in the Tarim Basin, hydrocarbon reservoirs in Ordovician rocks of the Yijianfang Formation contain large cavities (c. 10 m or more), vugs, fractures and porous fault rocks. Although some Yijianfang Formation outcrops contain shallow (formed near surface) palaeokarst features, cores from the Halahatang oilfield lack penetrative palaeokarst evidence. Outcrop palaeokarst cavities and opening-mode fractures are mostly mineral filled but some show evidence of secondary dissolution and fault rocks are locally highly (c. 30%) porous. Cores contain textural evidence of repeated formation of dissolution cavities and subsequent filling by cement. Calcite isotopic analyses indicate depths between c. 220 and 2000 m. Correlation of core and image logs shows abundant cement-filled vugs associated with decametre-scale fractured zones with open cavities that host hydrocarbons. A Sm–Nd isochron age of 400 ± 37 Ma for fracture-filling fluorite indicates that cavities in core formed and were partially cemented prior to the Carboniferous, predating Permian oil emplacement. Repeated creation and filling of vugs, timing constraints and the association of vugs with large cavities suggest dissolution related to fractures and faults. In the current high-strain-rate regime, corroborated by velocity gradient tensor analysis of global positioning system (GPS) data, rapid horizontal extension could promote connection of porous and/or solution-enlarged fault rock, fractures and cavities. Supplementary material: Stable isotopic analyses and the velocity gradient tensor and principal direction and magnitude calculation are available at https://doi.org/10.6084/m9.figshare.c.4946046 Thematic collection: This article is part of the The Geology of Fractured Reservoirs collection available at: https://www. lyellcollection.org/cc/the-geology-of-fractured-reservoirs Received 26 September 2019; revised 3 March 2020; accepted 8 April 2020

Hydrocarbon reservoirs in carbonate rocks in which pores and phenomena. Dissolution along fractures, for example, is widely fractures have been enlarged by dissolution are common in many reported (Nelson 1985; Narr et al. 2006). If deep-seated petroliferous basins in the world (Loucks 1999; Ahr 2011; Garland (mesogenetic; Choquette and Pray 1970) dissolution is important et al. 2012). These reservoirs typically have low to moderate in a given reservoir, predictions based on shallow (epigenetic) recovery efficiencies (Wardlaw and Cassan 1978) owing to dissolution and palaeokarst processes may be misleading. The heterogeneity and anisotropy in the size, spatial arrangement, and possibility of large-scale (volumetrically significant) dissolution at connectivity of open pores and fractures. The behaviour of such great depth (>1 km) has been doubted (e.g. Ehrenberg 2006; reservoirs is notoriously challenging to characterize and predict. In Ehrenberg et al. 2012). But workers agree that discerning cavities some instances, a palaeokarst model is a useful paradigm for formed by shallow (epigenetic) from those owing to deep-seated predicting open pores and fractures. In modern karst settings, (mesogenetic) dissolution is challenging, as by definition the corrosive surficial conditions and groundwaters form various cavities themselves preserve scant evidence of their origin. subsurface and geomorphological (landscape) features, including In this study, we use observations from outcrops and core to show caves (e.g. Ford and Williams 1989). Some of these features that fracture-related, deep dissolution and porous fault rock are collapse and coalesce during subsequent burial (Lohmann 1988; widespread in Yijianfang Formation rocks. Evidence from the Mazzullo & Harris 1991; James and Choquette 2012). Such Keping Uplift outcrops, 100 km SW from producing Yijianfang palaeokarst features are typically marked by evidence of near- Formation carbonate rocks in the Halahatang oilfield show that surface conditions at the time of formation, including cave-fill multiple stages of fracture-related dissolution are common in exposed sediments and distributions that match palaeotopography. Middle Ordovician rocks. Stable isotopic analyses indicate cavity- Palaeokarst porosity is reduced by cavity-fill sediments, compaction filling cements precipitated from fluids at depths of c.220 –2000 m. (stylolites) and mineralization. Mineral fills might not be diagnostic Crosscutting relations and abutting show that some fractures and vugs of porosity formation at shallow depths, as cements may accumulate formed and became cement filled prior to later dissolution events and during subsequent burial or hydrothermal fluid flow. Such formation of subsequent generations of fractures and vugs. Together reservoirs are widely recognized in China (e.g. Tian et al. 2016). these relations show that dissolution (probably multiple instances), However, deep-seated dissolution and cavity formation (e.g. not only cementation, occurred at depth. Giles and de Boer 1990; Vahrenkamp et al. 2004) as well as In core, a key isochron age from a fluorite-bearing fracture that structural processes create pore space in carbonate rocks, and postdates several types and stages of mineral-filled vugs (dissol- cavities and cavity-filling minerals may both be deep burial ution pores) provides evidence that substantial dissolution had

© 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/ licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

E. Ukar et al. occurred in Middle Ordovician carbonates by the end of the orogeny (290–250 Ma), transition into a foreland basin (forebulge) Devonian. We present a geological model based on our data and in the Triassic–Jurassic during the Indo-China orogeny, reported structural, petrological, geochemical, geophysical, petro- Cretaceous–Neogene extension related to the Yanshan orogeny, physical, radiometric and kinematic findings to show the evolution and late Neogene–Quaternary rapid subsidence (foreland basin) of the mechanisms and timing of dissolution that led to the during the Himalayan orogeny (Zhu et al. 2013b, 2017, 2019; formation of cavities in the Yijianfang Formation. The estimated Fig. 2). The Halahatang oilfield is located in the Halahatang fault-related porosity volume is at least c. 0.5%, which is augmented Depression and it is bordered by the Luntai Fault to the north, the by fault-related mesogenetic dissolution and fault-transverse Shuntuoguole Low Uplift to the south, the Lunnan Low to the east, extension. and the Yingmaili Low Uplift to the west (Zhu et al. 2012; Chang et al. 2013). Other important oilfields in the Tabei Uplift include the Tahe and Lunnan oilfields to the east (Chang et al. 2013)(Fig. 1). Geological setting During the Cambrian, dolomitic sequences with intercalated gypsum and salt layers were deposited in an array of half-grabens The Halahatang oilfield is part of the Tabei Uplift, one of the most developed as a result of regional extension (Lin et al. 2012; Gao and prolific hydrocarbon accumulation zones in the Tarim Basin (Li Fan 2013)(Fig. 2). Throughout the Ordovician, thick reef and shoal et al. 1996; Jia 1997; Pang et al. 2010)(Fig. 1). The Tabei Uplift has deposits accumulated in intra-platform, platform-margin and slope the largest proven oil/gas reserves of the Tarim Basin with over environments in a series of uplifts (i.e. Tazhong–Bachu, Tabei uplifts) 3 billion tons oil-equivalent (c. 21.5 billion barrels oil-equivalent) with intervening depressions (i.e. Kuqa, Awati, Manjiaer) (Li et al. discovered in Ordovician carbonate rocks currently at 6000–7000 m 1996; Jia 1997; Kang 2003; Gao and Fan 2013). In the Late depth (Zhu et al. 2016). Recently, giant oil accumulations have been Ordovician, the carbonate platform was drowned and overlain by a discovered in the lower Paleozoic section at depths >8000 m (Zhu thick section of deepwater continental shelf- and slope-facies of dark et al. 2019). mudstone and carbonaceous mudstone. The Silurian–Devonian The Tabei Uplift is an inherited structural high formed on the pre- was characterized by widespread marine sandstone deposition in Sinian metamorphic basement (Zou 2012; Zhu et al. 2013a). This intracratonic contractional basins (Liu et al. 2013; Zhao et al. area experienced Sinian–Ordovician rifting during the Caledonian 2016), giving way to alternate marine–nonmarine deposition in the orogeny, followed by Silurian–Permian uplift during the Hercynian

Fig. 1. (a) Simplified tectonic map of the Tarim Basin showing uplifts and depressions (after Zhao et al. 2014). Dashed lines mark boundaries of tectonic regions. Rectangle shows area with the selected 98 GPS stations between 77 and 81°E and between 39 and 44°N, from the northern Tarim Basin across the Tian Shan (after Zubovich et al. 2010). (b) Main oilfields in Halahatang Depression and Lunnan Uplift (after Chang et al. 2017a). 1, Halahatang; 2, Tahe; 3, Lunnan. Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

Deep-seated dissolution in the Halahatang oilfield

Fractures and vugs in core In the carbonate reservoirs studied, porosity does not correlate with permeability (Zhu et al. 2019). Such rocks constitute Type 1 ‘fracture type’ and ‘dissolution vug–fracture type’ or ‘fracture– cavity’ reservoirs, with very low porosity and permeability where most of the reservoir permeability is controlled by fractures and vugs (Nelson 1985; Bagrintseva et al. 1989). Based on petrological, cross-cutting, SEM-EDS, cathodoluminescence (CL), isotopic, and fluid inclusion thermometric and compositional analyses, Baqués et al. (2020) identified seven main groups of fractures (F) and associated vugs (V) in Halahatang Middle Ordovician carbonate core samples (group 1 oldest, group 7 youngest; Figs 2 and 3). Fractures and vugs were grouped on the basis of cross-cutting relationships, mineral fill and isotopic compositions irrespective of their orientations because individual fractures are highly variable in orientation so that the latter is not a good measure of timing (set) and/or type. Fractures show a spectrum of cement-fill degree that is dependent on size. Consequently, the conventional terms ‘vein’ and ‘joint’ are not helpful. We use the unambiguous descriptive term ‘fracture’ to refer to opening-mode fractures and specify the cement attributes where necessary. The oldest group of fractures (F1) comprises bed-perpendicular and bed-oblique, compacted (distorted) fractures filled with calcite cement that grades from equant to palisade morphology (Fig. 3a). Some F1 fractures contain a small amount of internal yellow carbonate sediment. The second group of fractures (F2) consists of bed-perpendicular, oblique and subhorizontal fractures filled with blocky calcite (Fig. 3a). F3 fractures are bed-perpendicular and oblique, calcite and bitumen-filled (Fig. 3b), and may preserve as much as 20% partial open porosity in fractures c.1mmwide.A fourth group (F4) encompasses bed-perpendicular and oblique fractures filled with calcite ± bitumen ± minor late pyrite cement (Fig. 3c). Some F4 are partially corroded. F5 fractures are infilled by euhedral fluorite, barite, celestite and calcite cements in variable proportions, and are associated with calcite, celestite and/or calcite filled silica host-rock replacement (Fig. 3d). F6 fractures are calcite- Fig. 2. Stratigraphy and lithology of Paleozoic–Cenozoic sediments in the filled (Fig. 3e) and show elevated 87Sr/86Sr isotopic ratios compared Tarim Basin, showing main unconformities and tectonic orogenies with the host rock. Finally, group 7 comprises bedding-oblique, (modified from Chang et al. 2013 and Zhao et al. 2014). Fracture (F) and stylolitic and sheared fractures (F7) containing insoluble residual vug (V) types as reported by Baqués et al. (2020). material, bitumen, dolomite and sheared calcite cement (Fig. 3f). Some fractures in this category contain cement bridges and crack-seal Carboniferous–Permian (Zhang et al. 1983; Liu and Xiong 1991). textures indicating cementation synchronous with fracture opening Almost continuous Paleozoic sedimentation was interrupted by (see Lander & Laubach 2015; Ukar & Laubach 2016; Baqués et al. sporadic volcanism, including the Permian flood basalt magma- 2020)(Fig. 3g). Such fractures may preserve up to 25% porosity. tism of the Tarim Large Igneous Province (e.g. Xu et al. 2014). Several dissolution features are evident in core including bed- From the Triassic to the Quaternary, the Tarim Basin has been parallel (S1) to bed-perpendicular stylolites (S2) (Fig. 3b, g and i), dominated by terrestrial siliciclastic deposition (Jia et al. 1995). an early diagenetic alteration of the host rock (‘mottle fabrics’) Hydrocarbon reservoirs in the Tabei Uplift mainly occur in Middle (Fig. 3h), and several types of vugs (dissolution pores larger than the Ordovician Yingshan and Yijianfang Formations in the Lunnan Low average grains (c. 20 µm)) up to 2 mm in diameter (Fig. 3b–i) Uplift and Halahatang Depression areas (Zhu et al. 2016, 2017) (Baqués et al. 2020). Vugs are usually associated with fractures and (Fig. 1). Muddy limestones and mudstones of the Tumuxiuke, an increased abundance of both occurs where the core is highly Lianglitage and Sangtamu Formations form the caprocks of these broken (bagged pieces) and FMI show decimetre- to decametre- reservoirs. scale fractured zones adjacent to open cavities (Fig. 4). The earliest generation of vugs (V1) are infilled by carbonate sediment having – Results mottled textures (Fig. 3h), whereas subsequently formed V2 V7 (youngest) vugs (Fig. 3b, c, g and i) are mainly infilled by blocky This study builds on analyses of 227 hand samples from 16 cores calcite cement with different isotopic and cathodoluminescence and visual inspection of c. 450 m of full-bore formation characteristics similar to those of fracture cements (see Baqués et al. microimages (FMI) and log data of the studied cores (Baqués 2020). The most widespread and abundant type of vugs (V3) are et al. 2020). Baques et al. reported an in-depth description of the filled with calcite and bitumen, and are usually associated with F3 diagenetic history recorded in these cores by means of detailed fractures, stylolites and V1 mottled fabrics, highlighting the petrographic, isotopic and fluid inclusion analyses of fracture and connection between fractures and dissolution in these rocks vug cements. Here we focus on constraining the timing of each (Fig. 3h and i). We did not observe vugs associated with F5 diagenetic event in the microstructural sequence and tie them to fractures, but based on the fact that this association is common for all outcrop observations and the geological history of the area (see other identified fracture and vug groups, V5 vugs are probable. Appendix A for methods). Similar to vugs, stylolites, both bed-parallel and tectonic, are Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

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Fig. 3. Plane-light optical photomicrographs of fractures and vugs in core. (a) F1 fracture filled with equant to palisade calcite cement cut by an F2 fracture filled with blocky cement. (b) V2 vug filled with blocky calcite cement cut by an F3 fracture filled with calcite and bitumen. Both terminate at a bed- parallel stylolite (S1) containing bitumen. (c) Pyrite-bearing F4 fracture associated with a V4 vug filled with calcite (+ bitumen + minor late pyrite, outside of the field of view) cement. (d) F5 fracture infilled by euhedral fluorite, barite, celestite and calcite cement associated with silica host-rock replacement. (e) F6 fracture cutting F4 fractures filled by calcite cement. (f) Bedding-oblique, stylolitic and sheared F7 fractures containing insoluble material, bitumen, dolomite and calcite cement cutting and displacing a cemented F4 fracture. (g) V7 vugs cutting a F7 fracture partially filled with synkinematic calcite bridges. Porosity-bearing F7 fractures may have acted as corrosive fluid conduits. Group 7 fractures and vugs terminate against an S2 stylolite. (h) V1 vugs infilled by carbonate sediment (mottle fabrics) cut by F3 fractures. (i) V3 vugs lined by calcite and later filled by bitumen. Vugs terminate against or are cut by bitumen-bearing S1 stylolites. ubiquitous and formed throughout the geological history of these reported hydrothermal activity in the area (e.g. Liu et al. 2017a,b)and rocks. All stylolites contain bitumen and some bed-parallel Mg-rich fluid infiltration, and their depth is undetermined. All stylolites also contain dolomite and replacement silica (Fig. 3). fracture and vug cements of group 3 (included) and younger contain Highly variable, but generally high (>80°C) homogenization primary inclusions of oil indicating the presence of hydrocarbons in temperatures of fluid inclusions within fracture and vug cements, as the system at the time of their filling by cement. well as high salinities (5–20% NaCl equiv.) indicate that fluid inclusions probably re-equilibrated with high-temperature burial Sm–Nd fluorite date from core fluids and do not provide information on the true temperature and depth of precipitation of the cements. Consequently, Baqués et al. To decipher timing of fluorite mineralization in the Halahatang (2020) determined fracture and vug cementation depths on the basis oilfield, we conducted Sm–Nd radiometric dating of fluorite, barite of stable isotopic analyses (Craig 1965; Machel 1999) assuming a and calcite aliquots of one F5 fracture fill from core RP4 (Fig. 5). geothermal gradient of 2.79°C per 100 m for the Tabei Uplift during Fluorite typically contains elevated REE concentrations that may the Middle Ordovician (Wang et al. 2014). Because cemented show sufficient variations in Sm/Nd ratios to allow dating using the fractures and vugs are cross-cut by the next generation of filled isochron approach (Turner et al. 2003). The 147Sm/144Nd ratios of the fractures and vugs, the calculated depths of cement precipitation samples analysed range from 0.083 (calcite) to 0.14 (fluorite + based on stable isotopic analyses represent dissolution depths. Host calcite + barite; whole fracture). All five analysed samples plot in a rock diagenesis, including mottled fabrics (V1), developed at linear array and yield an age of 400 ± 37 Ma, with an initial εNd value <220 m, within the shallow burial diagenetic environment, whereas of 0.511566 ± 0.000025 (2σ,MSWD=0.78)(Table 1; Fig. 5). F1 fractures formed and were cemented near the surface within the marine–phreatic level. Group 2 fractures and vugs (F2, V2) Fractures, faults and dissolution vugs in outcrop developed within the shallow burial diagenetic environment (c. 220 m), group 3 within the intermediate burial diagenetic Redissolved palaeokarst environment (c. 625 m) and group 4 within the deep burial diagenetic Structural diagenetic features in outcrops in the Bachu–Keping environment (c. 2000 m). Groups 5–7 are probably the result of Uplift region, western Tarim Basin (Fig. 1), share many Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

Deep-seated dissolution in the Halahatang oilfield

Fig. 4. (a) Stratigraphic section and FMI from RP7 core (see Fig. 11 for location of well). (b) Hand sample and plane-light optical photomicrograph showing a highly fractured interval. Sample is from the highly fractured area, preserved in bags because of the multiple centimetre- to decimetre-scale fragments. Highly fractured and dissolution-enhanced area shown in FMI was unrecovered by core. characteristics with Ordovician rocks in core from producing parts bright-red-luminescent subhedral calcite crystals (Fig. 6d). Silica of the basin, although with some important differences. Karstic and barite mineralization postdate calcite overgrowths. These palaeotopography (karst towers and dolines) and cave-fill sediments phases probably reflect interaction with silica-rich fluids that (chaotic breccias and laminated cave-sediment fill) are present in percolated through overlying Silurian siliciclastic rocks or high- the topmost section of the Yingshan Formation in the XikeEr temperature fluids (Fig. 6e). The palaeokarst is affected by outcrop, indicating that subaerial exposure and karstification redissolution that is best developed on the top surface of the occurred prior to deposition of unconformable Silurian red beds karstic system (karst towers) (Fig. 6f), forming vugs and cavities (Fig. 6a). Approximately 60 m below the unconformity, large that are partially cemented by palisade calcite crystals that can be polymictic breccia bodies are exposed. The breccia clasts consist of over 30 cm thick (Fig. 6g). Such vugs closely resemble those Ordovician carbonate clasts and the matrix consists of a reddish widespread in Halahatang cores. Oxygen and carbon isotopic muddy to sandy sediment (Fig. 6b). Cave breccias and cave analyses, especially the depleted values in δ18O compared with sediment fills show evidence of sedimentation and cementation in cave-sediment fill and host-rock calcite overgrowths within cave the vadose zone such as laminated and geopetal structures (Fig. 6c). infills and palisade calcite, indicate precipitation from higher- Laminated cave-sediment fillings consist of non-luminescent to temperature fluids (Fig. 7; Table 2). Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

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Fig. 5. (a) Photograph of RP4 core (see Fig. 11 for location of well) showing fluorite-, barite- and calcite-bearing fracture used for Sm–Nd radiometric dating. (b) 147Sm/144Nd v. 143Nd/144Nd isochron showing the results of the five analysed samples and the calculated age.

Porous and dissolution-enhanced faults for a considerable amount, perhaps as much as 50–60%, of modern In the Yijianfang and Keping outcrops (Fig. 1), opening-mode convergence between India and Eurasia (Zubovich et al. 2010). The fractures and centimetre- to decametre-scale faults are common rate of deformation near the Tian Shan exceeds that in the Himalaya (Figs 8 and 9). Crosscutting and abutting relations constrain a and Tibet, so the Tian Shan effectively marks the plate boundary relative time sequence of the fractures. Calcite-filled fractures between Eurasia and India. striking NNE–SSW are oldest, followed by east–west-, NW–SE- The velocity gradient tensor we determined from global position- and barren NNE–SSW-striking fractures (Fig. 8a and b). Thrust and ing system (GPS) data of Zubovich et al. (2010),(Appendix B) strike-slip faults strike predominantly N–S and NE–SW, respect- surrounding the Halahatang oilfield indicates that the minimum (most ively (Fig. 8c and d). At the Yijianfang outcrop, NW–SE-striking negative, most contractional) extension rate trends 174° and measures − −1 opening-mode fractures (Fig. 9a) have primary porosity and 36.5 nstrain a (Supplementary material), comparable in magni- – secondary vugs and caves similar to those in the XikeEr outcrop. tude with strain rates surrounding the Pacific North America plate Secondary caves are infilled by fluorite, coarse crystalline calcite, boundary in the California borderland (Shen et al. 2001). Positive and in some cases gypsum (Fig. 9b and c). Calcite δ18O and δ13C extension rate occurs in the intermediate principal direction, trending −1 values within NW–SE fractures are similar to those of the host rock, 084° and measuring 7.9 nstrain a , and the maximum principal rate −1 indicating high rock–fluid interaction (Fig. 7). These stable isotopic of extension is approximately vertical and measures 28.6 nstrain a . makeups resemble those of Middle Ordovician marine carbonates. Analysis was validated by comparing observed velocity data with In contrast, NE–SW strike-slip faults have damage and fault-core calculated velocities (Fig. 10). Model velocity vectors match −1 zones of highly porous and slightly calcite-cemented fault breccias observed velocities within 2 mm a (length of vectorial residual) with as much as 30% remaining macroscopic porosity (point-count at 94 out of 98 stations (Supplementary material; red rows highlight estimate) (Fig. 9d). Similar porous fault rocks are present along counter-examples). decametre-scale damage zones of NE–SW strike-slip faults in dolomitized Pengliaba Formation, within the Dabantagh outcrop Discussion (Fig. 9e and f). In these field examples, fault damage zones are several metres wide (Fig. 9e). The high-porosity rock is discon- Bright spots, bit drops, cavities and faults tinuous over c. 10 m, but the finite size of the outcrop (tens of Ordovician carbonate rocks in the Tabei Uplift have been described metres) precludes assessment of the continuity pattern. as thoroughly karsted (Wang et al. 1992; Gu 1999; Liu et al. 2004; Zou et al. 2009; Zeng et al. 2011a) on the basis of sudden increases Present-day strain rates in drilling rates, bit drops, blowouts and mud loss (Tian et al. 2016). In the Tahe oilfield, such drilling abnormalities have been The India–Eurasia collision drives roughly north–south contraction encountered in c. 39% of the total number of wells (He et al. in the Tarim Basin (e.g. Tapponnier and Molnar 1979; Heidbach 2010; Tian et al. 2016). Bit drops of c. 1 m are common in central et al. 2018). The northern Tarim Basin and the Tian Shan account (Tazhong) and northern Tarim Basin (Tabei; HA8, 2 m; HA9,

Table 1. Isotopic analyses of fluorite-, barite- and calcite-bearing F5 fracture used for the calculation of an isochron age

147Sm/144Nd ±2σ 143Nd/144Nd ±2σ [Nd] (ppm) ±2σ [Sm] (ppm) ±2σ RP4 10 WF 0.10959 0.00207 0.511844 0.000014 1.090 0.004 0.197 0.004 RP4-11 WF 0.14148 0.00076 0.511939 0.000012 1.200 0.005 0.281 0.001 RP4 10 B + C1 0.08657 0.00046 0.511798 0.000012 7.011 0.029 1.003 0.003 RP4 10 B + C2 0.08331 0.00045 0.511783 0.000012 5.784 0.024 0.797 0.003 RP4 10 B + C3 0.08681 0.00046 0.511793 0.000012 7.066 0.029 1.014 0.003 Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

Deep-seated dissolution in the Halahatang oilfield

Fig. 6. (a) Outcrop photograph showing karstic palaeotopography (karst towers and dolines) and collapse deformation structures, and karstic chaotic breccias present on the topmost section of the Yingshan Formation in the XikeEr outcrop. Silurian red beds unconformably overlie Ordovician carbonates, indicating subaerial exposure. (b) Polymictic breccia bodies exposed c. 60 m below the unconformity. (c) Cave-sediment fill showing laminated sediments and geopetal structures evidencing their deposition in the vadose zone. (d) Laminated cave-sediment fillings showing non-luminescent to bright-red- luminescent concentric calcite overgrowths as seen under optical cathodoluminescence (CL). (e) Silica and barite mineralization postdating calcite overgrowths. (f) Redissolution cavities and calcite precipitated in cavities developed at the top surface of the karstic system (karst towers). (g) Palisade calcite crystals, over 30 cm thick, precipitated in the remaining open porosity of the redissolved cavities.

1.5 m; Yang et al. 2012) and some recorded bit drops in the Tabei Zeng et al. 2011a ; Zhao et al. 2014). In the Lunnan field, karst Uplift are tens of metres (Yang et al. 2012; Zhao et al. 2014). Mud features occur mainly in the subsurface Yingshan Formation, which and drilling fluid losses before well completion may be upwards of is overlain directly by Carboniferous siliciclastic rocks (Zeng et al. 2300 m3 (Zhao et al. 2014; Tian et al. 2016). 2011a; Zhao et al. 2014), indicating a strong association of the karst Yingshan and Yijianfang Formations in northern and central is found with the unconformity. Evidence of collapsed and filled Tarim Basin are characterized by areas of anomalously high- caves are found in outcrops or strata in the reservoir interval, for amplitude seismic reflections (bright spots) at c. 4000–6500 m example, near Keping, where Ordovician carbonates are also depth (Zeng et al. 2011a, b; Zhao et al. 2014; Zhu et al. 2017; Yang directly overlain by Silurian red beds (Fig. 6a). et al. 2018) and the presence of bright spots has recently been Few large caves have been documented below 3 km, an confirmed in superdeep (>8000 m) Paleozoic reservoirs in southern observation that previously led some workers to conclude that Halahatang (Zhu et al. 2019). Bright spots are reflective of significant shallow palaeokarst cavities would not survive deep hydrocarbon-filled cavities that cause bit drops (Zeng et al. 2011a; burial (Loucks 1999). Evidence for substantial cavities in what are Yang et al. 2018; Zhu et al. 2019). Such cavities have been inferred clearly palaeokarst structures (e.g. Gao et al. 2018) shows that to be the main source of hydrocarbon storage and permeability and cavities do persist at depth. Also, rock tests and geomechanical are the main target for exploration (Yang et al. 2012; Zhao et al. modelling show that cavities can be stable under great loads, even in 2014; Zhu et al. 2019) with more than 80% success rate (Zeng et al. the absence of overpressure (e.g. Davis et al. 2017). Together, these 2011a). considerations show that shallow palaeokarst is a plausible Many researchers have interpreted such cavities to be the result explanation for the cavities encountered in the Halahatang oilfield. of palaeokarst (shallow or epigenetic dissolution) (e.g. Tian et al. Other evidence, however, suggests a more nuanced interpretation 2017; Xu et al. 2017; Zhang et al. 2018). Cavities having such an of cavities in the Halahatang oilfield. With the exception of the origin are typically marked by dissolution, sediment fillings, northernmost part of the area, a pervasive unconformity with chemical precipitates and localized fracturing, brecciation and Silurian siliciclastic rocks is absent in the palaeo-structurally lower collapse of cave walls and ceilings (Loucks 1999). In oilfields east Halahatang, indicating that subaerial exposure was short-lived and of Halahatang (Tahe, Lunnan), reservoirs with well-attested vadose–phreatic cave formation less likely compared with the up to palaeokarst features are widespread (Gu 1999; Liu et al. 2004; 120 myr hiatus to the NE (Liu et al. 2004; Zeng et al. 2011a; Zhao Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

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In outcrop, many NNE- and NNW-striking fault zones comprise porous fault rock, and dissolution along fractures formed caves lined with calcite and minerals precipitated from deep-sourced fluids (Fig. 9). Dissolution-enhanced fault rocks have been reported in core in the Halahatang area (Wu et al. 2019a). Both porous fault rock and dissolution-enhanced cavities could provide hydrocarbon storage and migration pathways in the subsurface. Based on the fragile character of the fault rock in outcrop, we infer that a bit encountering brecciated material in fault damage zones would sustain little resistance so drilling response might be indistinguish- able from a cavity-related bit drop. Both types of structure could be associated with the observed large drilling mud losses. Based on the association between small-scale vugs and fractures and metre- to decametre-scale fractured zones with open cavities observed in core and image logs (Fig. 4), we interpret these dissolution-enhanced features to be the small-scale equivalents of fractured zones and open cavities responsible for bright spots and bit drops. Corrosive agents such as acidic fluids associated with sulfur-rich oil, hydrothermal fluids, CO2-rich fluids derived from underlying strata, meteoric waters or a combination of these could migrate along fractures and faults and form the dissolution cavities observed in core and inferred from seismic data, FMIs and production data. Away from faults, such fluids would flow laterally, along the unconformity with overlying, impermeable Tumuxiuke Formation. A combination of these processes, fault-parallel and lateral flow, provides an explanation for bright spots localized along the Ordovician unconformity (Ning 2017; Wu et al. 2019b) as well as those that penetrate as far down as 800 m into the Yingshan Formation (Yang et al. 2018; Zhu et al. 2019). Based on our GPS analyses, Andersonian mechanics predict ideal movement on orthorhombic faults having mixed strike-slip kinematics (Reches and Dieterich 1983). Steeply dipping fractures that strike parallel to the Tian Shan (Figs 1 and 11a) should show rapid present-day horizontal contraction, whereas steeply dipping fractures at high angle to the Tian Shan should show active 18 13 Fig. 7. δ O and δ C cross-plots of host rocks and calcite cements extension. Conjugate components of strike-slip movement probably precipitated in vugs and fractures from XikeEr outcrop (a), Yijianfang and accompany both. Thus, our GPS analysis predicts that active Keping outcrops (b), and Dabantagh outcrop (c). Arrows show isotopic movement on steeply dipping discontinuities having both NNE and trends over time as established based on cross-cutting relationships. NNW strikes, the main orientations of pre-existing faults in Halahatang (Fig. 11a), is extensional if not actively opening. The calculated strain tensor agrees with in situ stress measure- et al. 2014). Seismic data lack the characteristic erosional ments at c. 7 km depth in the central part of the Tarim Basin south of topography and geomorphological patterns associated with uncon- our study area, which show that maximum extension is vertical (Sun formities such as sinuous fluvial channels and canyons, fluvial et al. 2017). In contrast to both, vertical maximum compression is valleys, sinkholes, and tower karsts and hills identified in typical of modern (non-tectonic) sedimentary basins, but here we documented karsted areas of the Tarim Basin (Zeng et al. 2011a). address an ancient (although now tectonic) sedimentary basin. Moreover, Halahatang oilfield cores lack pervasive sediment-filled Stress maps show north–south compression (e.g. Heidbach et al. cavities and breccias characteristic of subaerial palaeokarst (Fu 2018). Sun et al. (2017) reported in situ orientations of minimum 2019; Baqués et al. 2020). Although in the northern and compression c. N140–N170°, which would cause active extension central Halahatang bright spots are c.50–150 m beneath the top along NNE-striking faults. Rapid (recent and current) tectonic of the Middle Ordovician, they are also found away from the extension along high-angle pre-existing faults in strong Ordovician unconformity and into the underlying Yingshan Formation (Zhao carbonate rocks could lead to formation of wide extension fractures, et al. 2014; Ning 2017), providing further evidence for the lack of an especially near releasing bends of faults, such as those documented association between cavities and subaerial exposure soon after in carbonate rocks in Oman (e.g. Hilgers et al. 2006; Holland and deposition. Urai 2010). If rate of opening movement exceeds the rate at which Many bright spots are aligned with both NNE and NNW fault chemical processes can fill voids (calcite cementation) and/or if traces (Yang et al. 2012; Zhao et al. 2014; Ning 2017; Zhu et al. dissolution prevails in fractures, porosity and permeability along 2019). For those bright spots that appear unconnected or isolated, an fractures are likely to persist and increase through time. association with faults may not be ruled out because the apparent isolation may be the result of the low resolution of seismic data Cavity formation by protracted deep-seated dissolution (Zhao et al. 2014). Highest-yield wells are located near deep faults, with oil production increasing with increasing proximity to faults Because observations at Halahatang are inconsistent with palaeo- (Sun et al. 2015; Yin et al. 2016; Wu et al. 2016, 2019b; Lu et al. karst formed in the shallow subsurface, an effective geological 2017; Zhu et al. 2019). These observations are consistent with model for the mechanisms and timing of dissolution that led to the cavities formed by structural processes along faults and/or by deep- formation of cavities is necessary to guide exploration. Below we seated dissolution along faults. present a model of the geological evolution of the Yijianfang Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

Deep-seated dissolution in the Halahatang oilfield

Table 2. Stable isotopic analyses

Sample type Outcrop Formation Mineralogy δ13CVPDB δ18OVPDB

Yinshan Fm XikeEr Yingshan Calcite 0.19 −6.54 Yinshan Fm XikeEr Yingshan Calcite 0.36 −6.39 Laminated cave-sediment fill XikeEr Yingshan Non-pure calcite 0.33 −5.11 Calcite overgrowth XikeEr Yingshan Non-pure calcite −1.13 −9.92 Calcite overgrowth XikeEr Yingshan Non-pure calcite −0.53 −9.92 Calcite in vugs XikeEr Yingshan Calcite −1.09 −7.63 Calcite in vugs XikeEr Yingshan Calcite −0.37 −10.90 Palisade calcite in cavities XikeEr Yingshan Calcite −1.42 −13.29 Palisade calcite in cavities XikeEr Yingshan Calcite −1.52 −13.27 Pengliaba Fm Dabantagh Pengliaba Dolomite −1.71 −8.23 Dolomite cement in NE-SW fractures Dabantagh Pengliaba Dolomite −1.95 −8.66 Dolomite cement in NE-SW fractures Dabantagh Pengliaba Dolomite −0.50 −8.18 Dolomite cement within fault breccia Dabantagh Pengliaba Dolomite −1.96 −9.08 Calcite cement within fault breccia Dabantagh Pengliaba Calcite −1.43 −10.52 Yijianfang Fm Yijianfang Yijianfang Calcite 0.04 −6.57 Yijianfang Fm Yijianfang Yijianfang Calcite −0.22 −5.38 Yijianfang Fm Keping Yijianfang Calcite 0.78 −5.93 Calcite cement in NNE–SSW fractures Keping Yijianfang Calcite −1.12 −12.95 Calcite cement in NNE–SSW fractures Keping Yijianfang Calcite −1.17 −13.21 Calcite cement in NNE–SSW fractures Yijianfang Yijianfang Calcite −2.12 −12.08 Calcite cement in NW–SE fractures Yijianfang Yijianfang Calcite −0.24 −6.77 Calcite cement in NW–SE fractures Yijianfang Yijianfang Calcite 0.30 −6.17 Calcite cement in NW–SE fractures Yijianfang Yijianfang Calcite 1.02 −5.06 Calcite cement in NE–SW faults Yijianfang Yijianfang Calcite −0.85 −10.77 Calcite cement in NE–SW faults Keping Yijianfang Calcite −0.18 −8.47 Calcite cement in NE–SW faults Keping Yingshan Calcite −0.98 −11.24 Calcite cement in N–S faults Yijianfang Yijianfang Calcite 0.70 −5.92 Calcite cement in N–S faults Yijianfang Lianglitage Calcite 0.14 −5.71

Fig. 8. (a) Cross-cutting relationships between opening-mode fractures in the Yijianfang outcrop. (b) Late, barren NNE–SSW fractures. (c)NE–SW-striking thrust fault carrying Middle Ordovician Yijianfang Formation over Upper Ordovician Lianglitage Formation (Yijianfang outcrop) (Fig. 1). (d) Core of NE–SW strike-slip fault (Yijianfang outcrop). Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

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Fig. 9. (a)NW–SE-striking opening-mode fractures (Yijianfang outcrop) with secondary caves filled with fluorite and coarse-crystalline calcite and in some cases gypsum. (b) Plane-light optical photomicrograph showing fluorite and calcite cements precipitated in cavities. (c) Plane-light optical photomicrograph showing palisade gypsum cement precipitated along a fracture wall. (d)NE–SW strike-slip faults showing damage zones and fault cores of highly porous and weakly calcite-cemented fault breccias with as much as 30% residual porosity (Yijianfang outcrop). (e) Decametre-scale damage zones of NE–SW strike-slip faults in dolomitized Pengliaba Formation within the Dabantagh outcrop. (f) Porous fault rocks along damage zones.

Formation reservoir that integrates petrological, structural, geo- fractures (group 1; Fig. 3a and h) developed near the surface within chemical, geophysical, petrophysical, temporal (cross-cutting the marine–phreatic environment at this stage (Figs 12 and 13a). relations and Sm–Nd isotopic dating), outcrop analogue and Renewed uplift during the middle Caledonian orogeny was kinematic evidence summarized in this study. succeeded by a disconformity between the Lianglitage Formation The Yingshan and Yijianfang Formations were deposited in inner and overlying Sangtamu Formation (Fig. 2) following deposition in and middle-shelf to outer-shelf environments (Lin et al. 2012). a low- to moderate-energy environment (Lin et al. 2012; Zhao et al. During the middle Caledonian orogeny that resulted from the 2014). The western part of the Tabei palaeo-uplift experienced a collision of the Kunlun island arc and the Tarim plate (Yu et al. notable, regional angular unconformity and a network of caves and 2011), uplift and erosion caused subaerial exposure of the Yijianfang karstic breccias that are absent at Halahatang (Baqués et al. 2020). Formation and an angular unconformity with the overlying Group 2 fractures and vugs (Fig. 3a and b) formed during burial and Tumuxiuke Formation (Lin et al. 2012). In the Halahatang area, compaction and cements precipitated from marine to formation subaerial exposure was short-lived, resulting in the absence of fluids in the marine to shallow burial environment (c. 220 m) penetrative palaeokarst in our study area. Instead, incipient (Fig. 13b). At least some F2 probably follow similar orientations to dissolution with minor infiltration of soil sediment (V1) and F1 Caledonian faults, striking NNE and NNW (Cai et al. 2015). Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

Deep-seated dissolution in the Halahatang oilfield

Fig. 10. Validation plot comparing observed velocity data and velocity calculated using the new method described in Appendix B.

Following uplift, the Early Silurian sedimentary environment changed to nearshore and shallow-marine deposition. The first oil and gas accumulation phase occurred at this time (Lu et al. 2008b; Wang et al. 2008)(Fig. 12). Oil and gas migrated northward along unconformities and through faults and accumulated preferentially within Late Caledonian anticlinal traps (Zhu et al. 2013b). Fluids, including hydrocarbons and acidic fluids associated with sulfur-rich oil, flowed along group 3 fractures as indicated by the presence of oil inclusions in fracture calcite cement (Baqués et al. 2020), and laterally within Middle Ordovician strata, causing localized dissolution and formation of group 3 vugs and caves (Fig. 3b, 3h and 3i). Dissolution caused enlargement of dissolution-prone pre- Fig. 11. (a) Simplified map of main fault traces in the Halahatang area existing mottle fabrics. Calcite cements precipitated in fractures and (after Chang et al. 2017b and Wu et al. 2019b) used in the calculation of – vugs from formation fluids in the intermediate burial realm strain as measured along the marked east west scanline. Faults within the shaded area were projected onto the scanline. Numbers indicate picked (c. 625 m). faults. (b) Fault locations and lengths measured along the scanline shown An unconformity formed in the Devonian during the Early in (a). Hercynian orogeny (Li et al. 2010; Chen et al. 2012) leading to extensive karstification of Ordovician strata in the nearby Tahe and Lunnan oilfields (Li et al. 2011; Yan et al. 2011; Yang et al. 2014; Tian et al. 2016; Fig. 1). In the Halahatang area, the Yijianfang and fluorite cement confirms that widespread mesogenetic dissolution Yingshan Formations remained buried (c. 500–1000 m) and no cavities (V2–V4; Fig. 3b, c, h and e) existed in Middle Ordovician widespread subaerial karst developed. Fluids circulated along Early carbonates by the end of the Devonian (Figs 2 and 13c, d). Hercynian strike-slip faults and group 4 and 5 fractures (Fig. 3c to In the Carboniferous, regional unconformities formed between 3f). Owing to the impermeable nature of the Tumuxiuke Formation, the Carboniferous and underlying Silurian–Devonian strata in the fluids localized within Middle Ordovician strata flowed laterally, Manjia-er and Tabei area (Lu et al. 2008a; Zhu et al. 2012; Fig. 1). causing further dissolution and enlargement of vugs and caves and Fluvial and lacustrine facies were deposited in an interior basin precipitation of calcite cements within the deep burial environment setting (Zhu et al. 2012). Further cementation of previously formed (c. 2000 m; Baqués et al. 2020)(Fig. 13d). Corrosive fluids fractures and vugs probably occurred at this time. associated with precipitation of pyrite and/or fluorite (Esteban and The Permian was a period of extensive magmatism, both Taberner 2003) most likely caused partial dissolution of F4 extrusive and intrusive, in NW Tarim Basin (Yang et al. 2007; fractures. Tian et al. 2010; Xu et al. 2014; Shangguan et al. 2016) as well as The Sm–Nd age we report indicates that fluorite-bearing fractures associated normal faulting especially in the middle Permian (Liu (F5; Figs. 3d and 5a) filled at this time. Fluorite and other minerals et al. 2017b). Cements in group 6 fractures and vugs (Fig. 3e) show precipitated from high-temperature fluids present in cores in the depleted δ18O and δ13C and high 87Sr/86Sr similar to those reported Central Uplift (Tazhong; Fig. 1) and in outcrop in the Bachu– for Permian magmatism consistent with precipitation from Permian Keping region ( Fig. 9b) are thought to be linked to widespread hydrothermal fluids of igneous origin (Baqués et al. 2020) Permian magmatic activity (Jin et al. 2006; Wang et al. 2015)orto (Fig. 13e). low-temperature meteoric circular hydrothermal fluids in the late In the Hercynian (Permian), Early–Middle Ordovician source Yanshanian–Himalayan (Zhang et al. 2006), but this radiometric rocks entered the maximum hydrocarbon generation window in the age indicates that fluorite is younger at Halahatang. Our dated south of the Tabei area (Zhang et al. 2004; Zhang and Luo 2011; Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

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Fig. 12. Estimated timing and depths of formation of Groups (G) 1–7 fractures and vugs in the context of a burial history model calculated for one of the wells in our studied area (see Fig. 11a).

Zhu et al. 2013a, b, 2017; Ge et al. 2020; Fig. 12). This was the Himalayan orogeny (Guo 1993; Sobel and Dumitru 1997; Li et al. main period of hydrocarbon generation and expulsion from 2001). Pre-existing faults that strike NW–SE were reactivated in Cambrian–Ordovician source rocks in the Tarim Basin (Li et al. extension at this time (Li et al. 2013). Driven by thick Cretaceous– 2010; Ge et al. 2020). Oil migrated from the south to the north and Tertiary accumulations derived from the Tian Shan, Mid- to Late infilled previously formed vugs and cavities (Fig. 13e). In the latest Ordovician strata in the Halahatang Depression entered the Permian to earliest Triassic, thrust faults that penetrate pre-Jurassic hydrocarbon generation window, leading to a second phase of strata developed (Liu et al. 2017a). This new episode of reactivation hydrocarbon charge (Chang et al. 2013)(Fig. 12). Hydrocarbons along pre-existing faults probably caused further milling of porous migrated from the south to the north along faults and possibly also fault rock within fault damage zones, similar to that present in unconformities (Lu et al. 2008b). During the Pliocene–Quaternary, outcrop (Fig. 9e and f), so that the rocks became prone to the Halahatang Depression experienced rapid burial (Zhu et al. hydrocarbon storage. Main oil pools in Ordovician rocks were 2017). South of the study area, Cambrian-sourced oil remains formed in the Permian (Zhu et al. 2013a; Ge et al. 2020) so that uncracked because the residence time at temperatures greater than Hercynian tectonism is crucial to petroleum generation and c. 150°C has been relatively short (Zhu et al. 2018, 2019). accumulation in the Tarim Basin. Consistent with Heidbach et al. (2018), our velocity-gradient Following terrestrial sediment deposition in the Triassic (Lin tensor analysis shows that at present day, the maximum rate of et al. 2012; Zhu et al. 2013b), and uplift related to the late contraction is oriented roughly north–south and absolute extension Indosinian orogeny (Tang et al. 2014; Zhao et al. 2014), two persists along both steeply dipping fault orientations (NNE–SSW regional unconformities formed as a result of the Yanshanian and NNW–SSE) that dominate in the study volume (Fig. 11a). orogeny at the end of the Jurassic (Middle Yanshanian) and at the Progressive extension allows large cavities along faults to enlarge end of the Cretaceous (Later Yanshanian) (Tang et al. 2014) and stay connected at depth (Fig. 13g). Near-vertical fissures (Fig. 2). Although Meso-Cenozoic thrust faults dominate (Zhang formed under current conditions would connect pre-existing pores, et al. 1996; Sobel and Dumitru 1997; Yin et al. 1998; Bullen et al. vugs and caverns along faults and enhance reservoir permeability, 2003; Meng et al. 2008; Li et al. 2009, 2010; Zheng et al. 2009), the especially near releasing bends that evolved during fault segment Tarim Basin also experienced local extension in the Jurassic–Early linkage. Growth of the fracture network may have been promoted by Cretaceous when maximum compression was NE–SW, forming the corrosive fluids (Laubach et al. 2019). Although in northern left-lateral ENE and right lateral north–south transtensional fault Halahatang normal pressure systems indicate a well-developed zones (Li et al. 2013). Under NNE–SSW maximum principal fracture–cavity system, to the south, moderate overpressures with compression (σ1)(Li et al. 2013), pre-existing faults in this high pressure coefficients indicate that cavities remained isolated orientation would have experienced opening, providing further and with low connectivity (Zhu et al. 2019). volume for hydrocarbon storage. Group 7 sheared fractures (Fig. 3f) and tectonic stylolites and stylobreccias (Baqués et al. 2020) most Porosity estimate probably formed associated with this faulting, probably during a period of Mg-rich fluid upwelling as indicated by the presence of As a rough proxy for porosity along faults, we inferred opening dolomite (Baqués et al. 2020)(Figs 12 and 13f). strain accumulated by major (mapped) faults in the Halahatang area In the Late Cretaceous–late Cenozoic, conjugate NNE-striking along an east–west scanline across the study area (Fig. 11). Both right-lateral and east–west-striking left-lateral transtensional fault NNE- and NNW-striking faults were included under the assumption zones formed in the Tabei Uplift (Li et al. 2013) whereas that they are equally likely to contribute to fault-related strain. Field the direction of maximum compression was NE–SW during the observations (Fig. 9e) and subsurface mapping (Wu et al. 2019b) Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

Deep-seated dissolution in the Halahatang oilfield

Fig. 13. Continued.

Porosity was assumed to be as much as 30% of the fault material on the basis of limited examples from outcrop (Fig. 9d–f). This estimated fault porosity is within the mid to high end of the estimated volume of rock occupied by unconfined caves in carbonate aquifers (0.004–0.48%) (Worthington 1999; Worthington et al. 2000; Klimchouk 2006) and low end of those occupied by confined caves (c.1–5%) (Heward et al. 2000; Klimchouk 2006; Albert et al. 2015) in both shallow and deeply buried, collapsed karst (0.3–3%) (Weber and Bakker 1981, and Fig. 13. Evolution of fault- and fracture-related dissolution cavities in references therein). Reported porosity in Paleozoic reservoirs of the Ordovician Yijianfang Formation at the Halahatang oilfield. Oys, Tarim Basin (including karst) is 5–12% (Gu et al. 2002), suggesting Yingshan Formation; Oyj, Yijianfang Formation; Ol, Lianglitage that mesogenetic dissolution and mechanical openings such as Formation. Vertical exaggeration 17×. releasing bends along faults increased, by at least 10 times, total reservoir porosity. document that these faults have cores of considerable width. We Conclusions measured porous fault cores and damaged zones of 2.5 m for small- displacement (>10 m) faults (Fig. 9e). Longer faults may have fault Large cavities associated with high oil production and decametre- damage zones of up to 1100 m based on inferences from seismic scale bit drops in the Halahatang oilfield have been ascribed to and production data by Wu et al. (2019b). The relationship of fault shallow (epigenetic) dissolution and well-developed palaeokarst core width to fault length, if there is one, as reported by many that governs production from other Ordovician carbonate rocks in researchers (e.g. Walsh and Watterson 1987; La Bruna et al. 2018; the Tabei Uplift, where subaerial exposure during the Hercynian Scholz 2019), is unknown. As a crude approximation, we assumed orogeny was comparatively long lived. However, for the Halahatang that the fault length (in kilometres) corresponds to a fault-core width oilfield rocks, cavernous porosity probably formed at depths of c. (in metres) (see Robertson 1983; Scholz 1987; Hull 1988; Marrett 220–2000 m as a result of the flow of organic acidic, high- and Allmendinger 1990, and references therein). Fracture strain was temperature formation waters, and Mg-rich corrosive fluids along calculated as the sum of fault core widths divided by the length of fractures and especially faults. the scanline. Calculated fault-related strain is c. 0.4% (Table 3). This Core observations of open and filled vugs and fractures show that is a minimum estimate because only major faults were taken into the Halahatang area experienced several stages of fracture and account; addition of smaller, unmapped faults and faults that are associated dissolution marked by multiple generations of cross- below seismic resolution would increase, perhaps even double, this cutting fractures and vugs (large pores) containing cements with estimate. distinct petrological, isotopic and geochemical characteristics. Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

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Table 3. Mapped fault dimensions and calculated strain benefited from discussion with B. Loucks, Qilong Fu, Chaozhong Ning and L. Sivila, and the comments of journal referees J. Garland, V. La Bruna and an Distance east–west (km) Fault trace length (km) Average strike anonymous reviewer. Publication was authorized by the Director, Bureau of Economic Geology. 0.4 4.04 NE 5.65 14.79 NW 0.47 5.64 NE Funding This study was funded by CNPC-USA and CNPC-Tarim Oilfield Company, and by the Fracture Research and Application Consortium at the 0.4 10.97 NW Bureau of Economic Geology. Our work on fracture pattern development and 0.52 2.58 NE diagenesis is partly supported by grant DE-FG02-03ER15430 from Chemical 4.85 7.77 NE Sciences, Geosciences and Biosciences Division, Office of Basic Energy 0.66 3.84 NE Sciences, Office of Science, US Department of Energy. 0.95 6.1 NW 2.2 7.3 NE Author contributions EU: conceptualization (lead), data curation 0.38 2.51 NW (equal), funding acquisition (equal), investigation (equal), methodology 1.61 6.22 NW (equal), project administration (lead), supervision (lead), writing – original 2.65 7.19 NE draft (lead); VB: conceptualization (equal), data curation (lead), formal analysis (lead), investigation (equal), methodology (equal), writing – review & editing 1.23 2.38 NW (supporting); SEL: conceptualization (supporting), formal analysis (supporting), 3.03 2.27 NE funding acquisition (equal), methodology (supporting), writing – review & 0.64 15.07 NW editing (supporting); RM: conceptualization (supporting), formal analysis – 0.26 2.53 NW (supporting), writing original draft (supporting). 3.31 1.26 NE 0.17 6.26 NE Data availability statement All data generated or analysed during this 0.2 2.21 NE study are included in this published article (and its supplementary information 3.81 10.52 NE files). 3.81 21.52 NW 4.04 5 NW Scientific editing by Robert Holdsworth 2.25 8.65 NW 1.44 2.33 NE 0.95 17.25 NW Appendix A: Methods 0.71 6.47 NW Fieldwork was conducted in the Bachu–Keping Uplift region in the 2.68 western Tarim Basin. A total of 36 oriented carbonate rock samples 49.27 182.67 Total were collected from the Dabantagh, Yijianfang, XikeEr and Keping L construed as aperture (m) 182 outcrops (Bachu Uplift, NW Tarim Basin) (Fig. 1). Scanline length (m) 49452.67 Five fracture-filling fluorite, barite and calcite mineral Strain 0.0037 separates and whole-fracture aliquots from core RP4 were crushed to fine-grained powders in an agate mortar for radiometric dating. Mineral separates and whole-fracture Cemented fractures and vugs are cut by subsequent generations of powders were spiked with a mixed 150Nd–149Sm spike and structural diagenetic features indicating that episodes of both dissolved in alternating rounds of nitric acid, aqua regia, and dissolution and cementation occurred at mesogenetic depths. Based boric acid on a hotplate at 150°C at the class 100 Radiogenic on O–C isotopic values, and linking values to thermal history, vugs Isotope Clean Laboratory, The University of Texas at Austin. All may have formed at depths as great as 2 km. A Sm–Nd isochron age reagents used were either optima grade or double distilled. of 400 ± 37 Ma for fracture-filling fluorite indicates that some vugs Dissolution steps were repeated multiple times to achieve and probably associated dissolution-enhanced fault zones formed complete decomposition of more resistant mineral phases prior to the attested main period of hydrocarbon generation and (barite and fluorite). Rare earth elements (REE) were isolated expulsion in the late Hercynian (Permian). A complex tectonic using column chromatography by means of BioRad RE-Spec history resulted in multi-stage reactivation of faults both in shear and resin and nitric acid. Sm and Nd were isolated from the REE extension, and formation of porous fault rock and dissolution- fractions using BioRad LN Spec and dilute hydrochloric acid. enhanced cavities, as evidenced by fault rocks exposed in nearby Nd and Sm separates were analysed by thermal ionization mass outcrops. Estimates based on fault porosity and abundance and core spectrometry on a Triton system using double rhenium observations suggest that mesogenetic porosity enhancement of as filaments. An in-house macro was used to deconvolve the much as 5–10% is locally plausible. Values are probably much spike. The Excel macro contains the spike information and lower averaged over the entire Halahatang structure. Both porous iteratively corrects for spike and analytical mass bias until the fault rock and dissolution-enhanced cavities and fractures could corrections converge on the same value. Ages were calculated provide hydrocarbon storage and migration pathways and be using Isoplot 4. responsible for decametre-scale bit drops and large volumes of Aliquots of c. 0.3 mg were analysed for δ13C and δ18Ousinga drilling mud losses. ThermoFisher Scientific GasBench II coupled to a We used published GPS data to estimate deformation rates. ThermoFisher Scientific MAT 253 isotope ratio mass spectrom- Analysis indicates that, in agreement with previous work, the main eter (Révész and Landwehr 2002; Spötl and Vennemann 2003) faults in the Halahatang area (striking NNE–SSW and NNW–SSE) at The University of Texas at Austin. Samples were reacted in currently experience extension. Rapid extension promotes open helium-flushed vials with 103% H3PO4 at 50°C for 3 h. Data fractures connecting pre-existing pores, vugs and caverns. were calibrated using calcite standards NBS-18 (δ18OVPDB = −23.0‰, δ13CVPDB = −5.0‰) and NBS-19 (δ18OVPDB = −2.3‰, δ13CVPDB = 1.95‰). Twelve replicates of an internal carbonate standard were analysed throughout the analytical Acknowledgements We would like to thank S. Loewy and S. Hsia for – session to account for analytical drift and precision. Analytical Sm Nd geochronological dating, and T. Larson for stable isotopic analyses. We ‰ δ13 ‰ δ18 are grateful for access to the field and for fruitful discussions to Haijun Yang, precision of ±0.06 for CVPDB and ±0.10 for OVPDB Zhibin Huang, Chunyan Qi, Wenqing Pan, Kuanzhi Zhao and Yu Ye. This paper were routinely achieved. Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

Deep-seated dissolution in the Halahatang oilfield

To better quantify active deformation in the Halahatang equations, which yields oilfield, we analysed data published by Zubovich et al. (2010) P P P P 2 from geodetic GPS stations (N = 400+) representing 16 years of py pxvx pxpy pyvx rvxx ¼ P P P 2 2 2 observations. We limited data geographically to an area px py pxpy comprising 98 GPS stations (Fig. 1) to constrain a least- P P P P 2 squares estimate of the velocity gradient tensor, from which px pyvx pxpy pxvx rvxy ¼ P P P 2 2 2 principal directions and magnitudes of strain rate were px py pxpy determined (Appendix B). Using the analysis, present-day P P P P (B3) 2 kinematics were evaluated for orientations of faults and fractures py pxvy pxpy pyvy rvyx ¼ P P P 2 2 2 observed in outcrop, and in wells and seismic data. px py pxpy P P P P 2 px pyvy pxpy pxvy rvyy ¼ P P P : 2 2 2 Appendix B: Velocity gradient tensor and principal px py pxpy direction and magnitude calculation method It should be noted that rvxx and rvxy are independent of vy, whereas r r For approximately homogeneous deformation, the velocity gradient vyx and vyy are independent of vx. The velocity gradient tensor is r = r tensor (rv) relates the vector from the coordinate origin to the inherently asymmetric ( vxy vyx), so the anti-symmetric part location of a particle ( p) with the velocity vector (v) of the same quantifies average rotation rate about a vertical axis for the study particle via matrix multiplication (Ramsay and Huber 1983; area. The symmetric part of the velocity gradient tensor (i.e. strain Allmendinger et al. 2012): rate tensor) quantifies distortional and dilational components of deformation, and excludes the rotational component (Ramsay and Huber 1983; Allmendinger et al. 2012). Based on assumption that v ¼rv † p: (B1) the rate of volume change equals zero, the vertical rate of strain equals

Although the problem at hand is fundamentally three-dimensional, rvzz ¼(rvxx þrvyy) (B4) GPS data yield ineffective constraints in the vertical direction owing regardless of whether the vertical is a principal direction or not. If a to large uncertainties that stem from satellite geometry. For this principal direction is approximately vertical, then reason, we analyse only horizontal components of GPS data, and assume that the vertical is a principal direction of strain rate. The rvxz ¼rvzx ¼rvyz ¼rvzy ffi 0: (B5) assumption is justified by the footprint of the geodetic network, Conventional analysis of the velocity gradient tensor yields which is large in comparison with both the amplitudes of surface principal directions and rates of strain in all three spatial dimensions. topography and crustal thickness, so Anderson’s theory of faulting (Anderson 1951) predicts that the vertical should be a principal direction of stress. We apply equation (B1) in the horizontal plane, References and ignore curvature of the Earth’s surface. Provided that the rate of Ahr, W.M. 2011. Geology of Carbonate Reservoirs: the Identification, crustal volume change is zero, principal strain rates in the horizontal Description and Characterization of Hydrocarbon Reservoirs in Carbonate plane yield a complementary estimate of strain rate in the vertical Rocks. Wiley, New York. direction. Computation is simplified by demanding that v and p Albert, G., Virág, M. and Erőss, A. 2015. Karst porosity estimations from archive – observations share not only coordinate system but also units of cave surveys studies in the Buda Thermal Karst System (Hungary). International Journal of Speleology, 44, 151–165, https://doi.org/10.5038/ measure. 1827-806X.44.2.5 A local Cartesian coordinate system was defined by an origin Allmendinger, R.W., Cardozo, N. and Fisher, D.M. 2012. Structural Geology located at the centre of mass for GPS receiver coordinates in the Algorithms: Vectors and Tensors. Cambridge University Press, Cambridge. Anderson, E.M. 1951. The Dynamics of Faulting and Dyke Formation with study area, at which point model velocity equals the average velocity Applications to Britain. Hafner, New York. among all GPS stations. Averages were subtracted from published Bagrintseva, K.I., Dmitrievsky, A.N. and Bochko, R.A. 1989. Atlas of Carbonate values of v and p, and all length units were converted to kilometres. Reservoir Rocks of the Oil and Gas Fields of the East European and Siberian If deformation is approximately homogeneous, then each GPS Platforms. VNIGNI, Moscow, Russia. Baqués, V., Ukar, E., Laubach, S.E., Forstner, S.R. and Fall, A. 2020. Fracture, receiver (associated values of v and p) constitutes a partial constraint dissolution, and cementation events in middle Ordovician carbonate on a common velocity gradient tensor. In concert, the GPS dataset reservoirs. Tarim Basin, NW, China. Geofluids, 9037429, https://doi.org/10. (N = 98) dramatically over-constrains the velocity gradient tensor, 1155/2020/9037429. Bullen, M.E., Burbank, D.W. and Garver, J.I. 2003. Building the Northern Tien so we seek the unique solution that best fits the dataset in a least- Shan: Integrated Thermal, Structural, and Topographic Constraints. Journal of squares sense. The sums of component-wise squares for residuals Geology, 111, 149–165, https://doi.org/10.1086/345840 between model velocity (v0) and observed velocity (v) were Cai, C.F., Zhang, C. et al. 2015. Application of sulfur and carbon isotopes to oil– minimized across the scope of the dataset: source rock correlation: A case study from the Tazhong area, Tarim Basin, China. Organic Geochemistry, 83–84, 140–152, https://doi.org/10.1016/j. orggeochem.2015.03.012 Chang, X., Wang, T.-G., Li, Q. and Ou, G. 2013. Charging of Ordovician XN XN reservoirs in the Halahatang Depression (Tarim Basin, NW China) determined 0 2 2 – (v x vx) ¼ (rvxx px þrvxy py vx) by oil geochemistry. Journal of Petroleum Geology, 36, 383 398, https://doi. i¼1 i¼1 org/10.1111/jpg.12562 (B2) Chang, X., Mao, L., Xu, Y., Li, T. 2017a. C8 Light Hydrocarbon Parameters to XN XN 0 2 2 Indicate Source Facies and Thermal Maturity Characterized by (v y vy) ¼ (rvyx px þrvyy py vy) Comprehensive Two-Dimensional Gas Chromatography. Energy & Fuels, i¼1 i¼1 31, 9223–9231, https://doi.org/10.1021/acs.energyfuels.7b01595 Chang, X., Shi, B., Han, Z. and Li, T. 2017b.C5–C13 light hydrocarbons of crude oils from northern Halahatang oilfield (Tarim basin, NW China) where positive x and y signify geographical eastward and northward characterized by comprehensive two-dimensional gas chromatography. – directions, respectively. Minimization was achieved by setting Journal of Petroleum Science and Engineering, 157, 223 231, https://doi. org/10.1016/j.petrol.2017.07.043 partial derivatives to zero for each component of the velocity Chen, Q.L., Zhao, Y.Q., Li, G.R., Chu, C. and Wang, B. 2012. Features and gradient tensor, and solving the consequent system of four controlling factors of epigenic karstification of the Ordovician carbonates in Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

E. Ukar et al.

Akekule Arch, Tarim Basin. Journal of Earth Sciences, 23, 506–515, https:// Klimchouk, A.B. 2006. Unconfined versus confined speleogenetic settings: doi.org/10.1007/s12583-012-0271-4 variations of solution porosity. International Journal of Speleology, 35, Choquette, P W and Pray, L C. 1970. Geologic nomenclature and classification of 19–24, https://doi.org/10.5038/1827-806X.35.1.3 porosity in sedimentary carbonates. AAPG Bulletin, 54, 207–250. La Bruna, V., Agosta, F., Lamarche, J., Viseur, S. and Prosser, G. 2018. Fault Craig, H. 1965. The measurement of oxygen isotope paleotemperatures. In: growth mechanisms and scaling properties in foreland basin system: The case Tongiorgi, E. (ed.) Proceedings of the Spoleto Conference on Stable Isotopes study of Monte Alpi, Southern Apennines, Italy. Journal of Structural in Oceanographic Studies and Paleotemperatures, 161–182. Geology, 116,94–113, https://doi.org/10.1016/j.jsg.2018.08.009 Davis, T., Healy, D., Bubeck, A. and Walker, R. 2017. Stress concentrations Lander R.H. and Laubach, S.E. 2015. Insights into rates of fracture growth and around voids in three dimensions: The roots of failure. Journal of Structural sealing from a model for quartz cementation in fractured sandstones. GSA Geology, 102, 193–207, https://doi.org/10.1016/j.jsg.2017.07.013 Bulletin, 127, 516–538, https://doi.org/10.1130/B31092.1 Ehrenberg, S.N. 2006. Porosity destruction in carbonate platforms. Journal of Laubach, S.E., Lander, R.H., Criscenti, L.J. et al. 2019. The role of chemistry in Petroleum Geology, 29,41–52, https://doi.org/10.1111/j.1747-5457.2006. fracture pattern development and opportunities to advance interpretations of 00041.x geological materials. Reviews of Geophysics, 57, 1065–1111, https://doi.org/ Ehrenberg, S.N., Walderhaug, O. and Bjørlykke, K. 2012. Carbonate porosity 10.1029/2019RG000671 creation by mesogenetic dissolution: Reality or illusion? AAPG Bulletin, 96, Li, B.L., Guan, S.W. and Li, C.X. 2009. Paleo-Tectonic Evolution and 217–233, https://doi.org/10.1306/05031110187 Deformation Features of the Lower Uplift in the Central Tarim Basin. Esteban, M. and Taberner, C. 2003. Secondary porosity development during late Geological Review, 55, 521–530 [in Chinese with English abstract]. burial in carbonate reservoirs as a result of mixing and/or cooling of brines. Li, D., Liang, D., Jia, C., Wang, G., Wu, Q. and He, D. 1996. Hydrocarbon Journal of Geochemical Exploration, 78–79, 355–359, https://doi.org/10. accumulations in the Tarim basin, China. AAPG Bulletin, 80 , 1587–1603. 1016/S0375-6742(03)00111-0 Li, J.J., Fang, X.M., Zhao, Z.J. and Song, Y.G. 2001. Late Cenozoic intensive Ford, D.C. and Williams, P.W. 1989. Karst Geomorphology and Hydrology. Vol. uplift of Qinghai–Xizang Plateau and its impacts on environments in 601. Unwin Hyman, London. surrounding area. Quaternary Sciences, 21, 381–391. Fu, Q. 2019. Characterization and discrimination of paleokarst breccias and Li, K., Cai, C. et al. 2011. Origin of palaeo-waters in the Ordovician carbonates in pseudobreccias in carbonate rocks: insight from Ordovician strata in the Tahe oilfield, Tarim Basin: constraints from fluid inclusions and Sr, C and O northern Tarim Basin, China. Sedimentary Geology, 382,61–74, https://doi. isotopes. Geofluids, 11,71–86, https://doi.org/10.1111/j.1468-8123.2010.00312.x org/10.1016/j.sedgeo.2019.01.007 Li, M.J., Wang, T.G., Chen, J.F., He, F.Q., Yun, L., Akbar, S. and Zhang, W.B. Gao, D., Lin, C., Hu, M., Yang, H. and Huang, L. 2018. Paleokarst of the 2010. Paleo-heat flow evolution of the Tabei Uplift in Tarim Basin, northwest Lianglitage Formation related to tectonic unconformity at the top of the China. Journal of Asian Earth Sciences, 37,52–66, https://doi.org/10.1016/j. Ordovician in the eastern Tazhong Uplift, Tarim Basin, NW China. jseaes.2009.07.007 Geological Journal, 53, 458–474, https://doi.org/10.1002/gj.2902 Li, Y., Zhao, Y. and Zhang, Q.-R. 2013. Meso-Cenozoic extensional structures in Gao, Z.Q. and Fan, T.L. 2013. Ordovician intra-platform shoal reservoirs in the the Northern Tarim Basin, NW China. International Journal of Earth Tarim Basin, NW China: Characteristics and depositional controls. Bulletin of Sciences, 102, 1029–1043, https://doi.org/10.1007/s00531-012-0847-3 Canadian Petroleum Geology, 61,83–100, https://doi.org/10.2113/gscpgbull. Lin, C.S., Yang, H.J., Liu, J.Y., Rui, Z.F., Cai, Z.Z., Li, S.T. and Yu, B.S. 2012. 61.1.83 Sequence architecture and depositional evolution of the Ordovician carbonate Garland, J., Neilson, J.E., Laubach, S.E. and Whidden, K.J. 2012. Advances in platform margins in the Tarim Basin and its response to tectonism and sea- carbonate exploration and reservoir analysis. Geological Society, London, level change. Basin Research, 24, 559–582, https://doi.org/10.1111/j.1365- Special Publications, 370,1–15, https://doi.org/10.1144/SP370.15. 2117.2011.00536.x Ge, X., Shen, C., Selby, D., Feely, M. and Zhu, G. 2020. Petroleum evolution Liu, C.A. and Xiong, J.F. 1991. New understanding of Carboniferous and within the Tarim Basin, northwestern China: Insights from organic Permian stratigraphy in the northern region of Tarim basin. In: Jia, R. (ed.) geochemistry, fluid inclusions, and rhenium–osmium geochronology of the Research of petroleum geology of the northern Tarim basin in China, 1. Halahatang oil field. AAPG Bulletin, 104, 329–355, https://doi.org/10.1306/ University of Geoscience Press, Wuhan, 64–73. 05091917253 Liu, C.X., Xu, B.L. and Zhou, T.R. 2004. Petrochemistry and tectonic Giles, M.R. and de Boer, R.B. 1990. Origin and significance of redistributional significance of Hercynian alkaline rocks along the northern margin of the secondary porosity. Marine and Petroleum Geology, 7, 378–397, https://doi. Tarim platform and its adjacent area. Xinjiang Geology, 22,43–49 [in Chinese org/10.1016/0264-8172(90)90016-A with English abstract]. Gu, J., Jia, J. and Fang, H. 2002. Reservoir characteristics and genesis of high- Liu, J., Li, Z., Cheng, L. and Li, J. 2017a. Multiphase Calcite Cementation and porosity and high-permeability reservoirs in Tarim Basin. Chinese Science Fluids Evolution of a Deeply Buried Carbonate Reservoir in the Upper Bulletin, 47,12–19, https://doi.org/10.1007/BF02902813 Ordovician Lianglitage Formation, Tahe Oilfield, Tarim Basin, NW China. Gu, J.Y. 1999. Characteristics and evolutional model of karst reservoirs of lower Geofluids, 2017, 1–19, https://doi.org/10.1155/2017/4813235 Ordovician carbonate rocks in Lunnan area of Tarim Basin. Journal of Liu, L.H., Ma, Y.S., Liu, B. and Wang, C.L. 2017b. Hydrothermal dissolution of Palaeogeography, 1,54–60 [in Chinese with English abstract]. Ordovician carbonates rocks and its dissolution mechanism in Tarim Basin, Guo, J. 1993. Buried-hill paleokarst and its control on reservoir heterogeneity in China. Carbonates and Evaporites, 32, 525–537, https://doi.org/10.1007/ Ordovician, Lunnan region of Tarim Basin [in Chinese with English abstract]. s13146-016-0309-2 Acta Sedimentologica Sinica, 11,56–64. Liu, S., Su, S. and Zhang, G. 2013. Early Mesozoic basin development in North He, Z.L., Peng, S.T. and Zhang, T. 2010. Controlling factors and genetic pattern China: Indications of cratonic deformation. Journal of Asian Earth Sciences, of the Ordovician reservoirs in the Tahe area, Tarim Basin. Oil and Gas 62, 221–236, https://doi.org/10.1016/j.jseaes.2012.09.011 Geology, 31, 743–752 [in Chinese with English abstract]. Lohmann, K.C. 1988. Geochemical patterns of meteoric diagenetic systems and Heidbach, O., Rajabi, M. et al. 2018. The World Stress Map database release their application to studies of paleokarst. In: Paleokarst. Springer, New York, 2016: Crustal stress pattern across scales. Tectonophysics, 744, 484–498, 58–80. https://doi.org/10.1016/j.tecto.2018.07.007 Loucks, R.G. 1999. Paleocave carbonate reservoirs: Origins, burial depth Heward, A.P., Chuenbunchom, S., Makel, G., Marsland, D. and Spring, L. 2000. modifications, spatial complexity, and reservoir implications. AAPG Bulletin, Nang Nuan oil field, B6/27, Gulf of Thailand: karst reservoirs of meteoric or 83, 1795–1834. deep-burial origin? Petroleum Geoscience, 6,15–27, https://doi.org/10.1144/ Lu, X., Wang, Y. et al. 2017. New insights into the carbonate karstic fault system petgeo.6.1.15 and reservoir formation in the Southern Tahe area of the Tarim Basin. Marine Hilgers, C., Kirschner, D.L., Breton, J.P. and Urai, J.L. 2006. Fracture sealing and Petroleum Geology, 86, 587–605, https://doi.org/10.1016/j.marpetgeo. and fluid overpressures in limestones of the Jabal Akhdar dome, Oman 2017.06.023 mountains. Geofluids, 6, 168–184, https://doi.org/10.1111/j.1468-8123.2006. Lu, X., Yang, N., Zhou, X.Y., Yang, H.J. and Li, J.J. 2008a. Influence of 00141.x Ordovician carbonate reservoir beds in Tarim Basin by faulting. Science in Holland, M. and Urai, J.L. 2010. Evolution of anastomosing crack –seal vein China Series D: Earth Sciences, 51,53–60, https://doi.org/10.1007/s11430- networks in limestones: Insight from an exhumed high-pressure cell, Jabal 008-6016-7 Shams, Oman Mountains. Journal of Structural Geology, 32, 1279–1290, Lu, Y.H., Xiao, Z.Y., Gu, Q.Y. and Zhang, Q.C. 2008b. Geochemical https://doi.org/10.1016/j.jsg.2009.04.011 characteristics and accumulation of marine oil and gas around Halahatang Hull, J. 1988. Thickness–displacement relationships for deformation zones. depression, Tarim Basin, China. Science in China Series D, 51 , 195–206, Journal of Structural Geology, 10, 431–435, https://doi.org/10.1016/0191- https://doi.org/10.1007/s11430-008-5006-0 8141(88)90020-X Machel, H.G. 1999. Effects of groundwater flow on mineral diagenesis, with James, N.P. and Choquette, P.W. 2012. Paleokarst. Springer, Berlin. emphasis on carbonate aquifers. Hydrogeology Journal, 7,94–107, https:// Jia, C.Z. 1997. Structural Characteristics and Gas in Tarim Basin of China. doi.org/10.1007/s100400050182 Petroleum Industry Press, Beijing [in Chinese with English abstract]. Marrett, R. and Allmendinger, W. 1990. Kinematic analysis of fault-slip data. Jia, C.Z., Wei, G.Q., Yao, H.J. and Li, L.C. 1995. Tectonic Evolution of Tarim Journal of Structural Geology, 12, 973–986, https://doi.org/10.1016/0191– and Regional Tectonics. Petroleum Industry Press, Beijing [in Chinese]. 8141(90)90093-E Jin, Z., Zhu, D., Hu, W., Zhang, X., Wang, X. and Yan, X. 2006. Geological and Mazzullo, S.J. and Harris, P.M., 1991. An overview of dissolution porosity geochemical signatures of hydrothermal activity and their influence on carbonate development in the deep-burial environment, with examples from carbonate reservoir beds in the Tarim Basin. Acta Geologica Sinica, 80,245–253. reservoirs in the Permian Basin. In: M. Candelaria, (ed.) Permian Basin Plays - Kang, Z. 2003. Discovery and exploration of like-layered reservoir in Tahe Tomorrow's Technology Today. West Texas Geological Society (and Permian Oilfield of Tarim Basin. Acta Petrolei Sinica, 24,4–9. Basin Section SEPM), 91–89, 125–138. Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

Deep-seated dissolution in the Halahatang oilfield

Meng, Q.L., Li, Y., Shi, J., Jing, B., Feng, X. and Zhao, Y. 2008. Main characters carbonate platform, Luconia Province, offshore Sarawak, Malaysia. In: and active ages of the Serikbuya and Kangxi faults in the western Tarim basin. Eberli, G.P., Massaferro, J.L. and Sarg, R.F. (eds), Seismic imaging of Chinese Journal of Geology, 43, 282–293. carbonate reservoirs and systems, AAPG Memoir, 81, 329–350. Narr, W., Schechter, D.S. and Thompson, L.B. 2006. Naturally Fractured Walsh, J.J. and Watterson, J. 1987. Distributions of cumulative displacement and Reservoir Characterization,Vol. 112. Society of Petroleum Engineers, seismic slip on a single normal fault surface. Journal of Structural Geology, 9, Richardson, TX. 1039–1046, https://doi.org/10.1016/0191-8141(87)90012-5 Nelson, R.A. 1985. Geologic Analysis of Naturally Fractured Reservoirs. Gulf, Wang, Q.M., Nishidai, T. and Coward, M.P. 1992. The Tarim Basin, Houston, TX. northwestern China: Formation and aspects of petroleum geology. Journal Ning, C. 2017. Control of stratigraphic sequence on karst reservoirs: a case study of Petroleum Geology, 15,5 –34, https://doi.org/10.1111/j.1747-5457.1992. on the Ordovician carbonate reservoirs in the North Tarim Basin, China. tb00863.x Search and Discovery Article #10936. AAPG Annual Convention and Wang, T.-G., He, F., Wang, C., Zhang, W. and Wang, J. 2008. Oil filling history Exhibition, Houston, TX, April 2–5. of the Ordovician oil reservoir in the major part of the Tahe Oilfield, Tarim Pang, X., Tian, J., Pang, H., Xiang, C., Jiang, Z. and Li, S. 2010. Main progress Basin, NW China. Organic Geochemistry, 39, 1637–1646, https://doi.org/10. and problems in research on Ordovician hydrocarbon accumulation in the 1016/j.orggeochem.2008.05.006 Tarim Basin. Petroleum Science, 7, 147–163, https://doi.org/10.1007/s12182- Wang, Z., Yang, H., Qi, Y., Chen Y. and Xu, Y. 2014. Ordovician gas exploration 010-0022-z breakthrough in the Gucheng lower uplift of the Tarim Basin and its Ramsay, J.G. and Huber, M.I. 1983. The Techniques of Modern Structural enlightenment. Natural Gas Industry, B1,32–40, https://doi.org/10.1016/j. Geology: Strain Analysis (Vol. 1). Academic Press, London. ngib.2014.10.004 Reches, Z. and Dieterich, J. 1983. Faulting of rocks in a three-dimensional Wang, W., Li, J., Zhang, S., Wang, G., Yan, X. and Tian, W. 2015. U–Pb zircon strain field. I. Failure of rocks in polyaxial, servo-control experiments. geochronology of basal granite by LA-ICP-MS in Yitong Basin, Northeast Tectonophysics, 95, 111–132, https://doi.org/10.1016/0040-1951(83)90263-9 China: Implications for origin of limestone. Marine and Petroleum Geology, Révész, K.M. and Landwehr, J.M. 2002. δ13C and δ18O isotopic composition of 68, 583–595, https://doi.org/10.1016/j.marpetgeo.2014.08.001 CaCO3 measured by continuous flow isotope ratio mass spectrometry: Wardlaw, N.C. and Cassan, J.P. 1978. Estimation of recovery efficiency by visual statistical evaluation and verification by application to Devils Hole core DH- observation of pore systems in reservoir rocks. Bulletin of Canadian 11 calcite. Rapid Communications in Mass Spectrometry, 16, 2102–2114, Petroleum Geology, 26, 572–585. https://doi.org/10.1002/rcm.833 Weber, K.J. and Bakker, M. 1981. Fracture and vuggy porosity. Society of Robertson, E.C. 1983. Relationship of fault displacement to gouge and breccia Petroleum Engineers Annual Technical Conference, SPE preprint, 10332, San thickness. American Institute of Mining Engineers Transactions, 274, Antonio, TX. 1426–1432. Worthington, S.R.H. 1999. A comprehensive strategy for understanding flow in Scholz, C.H. 1987. Wear and gouge formation in brittle faulting. Geology, 15, carbonate aquifers. In: Palmer, A.N., Palmer, M.V. and Sasowsky, I.D. (eds) 493–495, https://doi.org/10.1130/0091-7613(1987)15<493:WAGFIB>2.0. Karst Modelling. Karst Waters Institute, Special Publication, 5. CO;2 Charlottesville, VA, 30–37. Scholz, C.H. 2019. The Mechanics of Earthquakes and Faulting. Cambridge Worthington, S.R.H., Ford, D.C. and Beddows, P.A. 2000. Porosity and University Press, Cambridge. permeability enhancement in unconfined carbonate aquifers as a result of Shangguan, S., Peate, I.U., Tian, W. and Xu, Y. 2016. Re-evaluating the solution. In: Klimchouk, A., Ford, D., Palmer, A. and Dreybrodt, W. (eds) geochronology of the Permian Tarim magmatic province: implications for Evolution of Karst Aquifers. National Speleological Society, Huntsville, AL, temporal evolution of magmatism. Journal of the Geological Society, London, 463–472. 173, 228–239, https://doi.org/10.1144/jgs2014-114 Wu, G., Yang, H., He, S., Cao, S., Liu, X. and Jing, B. 2016. Effects of structural Shen, Z.-K., Wang, M., Li, Y., Jackson, D.D., Yin, A., Dong, D. and Fang, P. segmentation and faulting on carbonate reservoir properties: a case study from 2001. Crustal deformation along the Altyn Tagh fault system, western China, the central uplift of the Tarim Basin, China. Marine and Petroleum Geology, from GPS. Journal of Geophysical Research, 106, 30607–30621, https://doi. 71, 183–197, https://doi.org/10.1016/j.marpetgeo.2015.12.008 org/10.1029/2001JB000349 Wu, G., Gao, L., Zhang, Y., Ning, C. and Xie, E. 2019a. Fracture attributes in Sobel, E.R. and Dumitru, T.A. 1997. Thrusting and exhumation around the reservoir-scale carbonate fault damage zones and implications for damage margins of the western Tarim Basin during the India–Asia collision. zone width and growth in the deep subsurface. Journal of Structural Geology, Journal of Geophysics Research, 102, 5043–5063, https://doi.org/10.1029/ 118, 181–193, https://doi.org/10.1016/j.jsg.2018.10.008 96JB03267 Wu, G., Xie, E., Zhang, Y., Qing, H., Luo, X. and Sun, C. 2019b. Structural Spötl, C. and Vennemann, T.W. 2003. Continuous-flow isotope ratio mass diagenesis in carbonate rocks as identified in fault damage zones in the spectrometric analysis of carbonate minerals. Rapid Communications in Mass northern Tarim Basin, NW China. Minerals, 9, 360–374, https://doi.org/10. Spectrometry, 17, 1004–1006, https://doi.org/10.1002/rcm.1010 3390/min9060360 Sun, D., Yang, L.S. et al. 2015. Strike-slip fault system in Halahatang area of Xu, X., Chen, Q., Chu, C., Li, G., Liu, C. and Shi, Z. 2017. Tectonic evolution Tarim Basin and its control on reservoirs of Ordovician marine carbonate rock. and paleokarstification of carbonate rocks in the Paleozoic Tarim Basin. Natural Gas Geoscience, 26,80–87 [in Chinese with English abstract]. Carbonates and Evaporites, 32, 487–496, https://doi.org/10.1007/s13146- Sun, D., Sone, H., Lin, W., Cui, J., He, B., Lv, H. and Cao, Z. 2017. Stress state 016-0307-4 measured at ∼7 km depth in the Tarim Basin, NW China. Scientific Reports, 7, Xu, Y.G., Wei, X., Luo, Z.Y., Liu, H.Q. and Cao, J. 2014. The Early Permian 4503, https://doi.org/10.1038/s41598-017-04516-9 Tarim Large Igneous Province: main characteristics and a plume Tang, L.J., Huang, T. et al. 2014. Fault systems and their mechanisms of the incubation model. Lithos, 204,20–35, https://doi.org/10.1016/j.lithos.2014. formation and distribution of the Tarim Basin, NW China. Journal of Earth 02.015 Science, 25, 169–182, https://doi.org/10.1007/s12583-014-0410-1 Yan, W., Wang, X., Zhang, T., Ding, Y., Liu, C. and Lü, H. 2011. Investigations Tapponnier, P. and Molnar, P. 1979. Active faulting and Cenozoic tectonics of the of the karst during the Episode III of the mid-Caledonian in the Tahe Oilfield. Tien Shan, Mongolia, and Baykal Regions. Journal of Geophysical Research, Acta Petrolei Sinica, 32, 411–416. 84, 3425–3459, https://doi.org/10.1029/JB084iB07p03425 Yang, H., Zhu, G., Yang, Y., Su, J. and Zhang, B. 2014. The geological Tian, F., Jin, Q. et al. 2016. Multi-layered Ordovician paleokarst reservoir characteristics of reservoirs and major controlling factors of hydrocarbon detection and spatial delineation: A case study in the Tahe Oilfield, Tarim accumulations in the Ordovician of Tazhong area, Tarim Basin. Energy Basin, Western China. Marine and Petroleum Geology, 69,53–73, https://doi. Exploration and Exploitation, 32, 345–368, https://doi.org/10.1260/0144- org/10.1016/j.marpetgeo.2015.10.015 5987.32.2.345 Tian, F., Lu, X., Zheng, S., Zhang, H., Rong, Y., Yang, D. and Liu, N. 2017. Yang, P., Sun, S.Z., Liu, Y., Li, H., Dan, G. and Jia, H. 2012. Origin and Structure and filling characteristics of paleokarst reservoirs in the northern architecture of fractured–cavernous carbonate reservoirs and their influences Tarim Basin, revealed by outcrop, core and borehole images. Open on seismic amplitudes. Leading Edge, 31, 140–150, https://doi.org/10.1190/1. Geoscience, 9, 266–280, https://doi.org/10.1515/geo-2017-0022 3686911 Tian, W., Campbell, I.H. et al. 2010. The Tarim picrite–basalt rhyolite suite, a Yang, S.F., Li, Z., Chen, H., Santosh, M., Dong, C.W. and Yu, X. 2007. Permian Permian flood basalt from northwest China with contrasting rhyolites bimodal dyke of Tarim Basin, NW China: Geochemical characteristics and produced by fractional crystallization and anatexis. Contributions to tectonic implications. Gondwana Research, 12, 113–120, https://doi.org/10. Mineralogy and Petrology, 160, 407–425, https://doi.org/10.1007/s00410- 1016/j.gr.2006.10.018 009-0485-3 Yang, W., Gong, X. and Li, W. 2018. Geological origins of seismic bright spot Turner, S., George, R., Jerram, D.A., Carpenter, N. and Hawkesworth, C. 2003. reflections in the Ordovician strata in the Halahatang area of the Tarim Basin, Case studies of plagioclase growth and residence times in island arc lavas from western China. Canadian Journal of Earth Sciences, 55, 1297–1311, https:// Tonga and the Lesser Antilles, and a model to reconcile discordant age doi.org/10.1139/cjes-2018-0055 information. Earth and Planetary Science Letters, 214, 279–294, https://doi. Yin, A., Nie, S., Craig, P., Harrison, T.M., Ryerson, F.J., Qian, X. and Yang, G. org/10.1016/S0012-821X(03)00376-5 1998. Late Cenozoic tectonic evolution of the southern Chinese Tian Shan. Ukar E. and Laubach S.E. 2016. Syn- and postkinematic cement textures in Tectonics, 17,1–27, https://doi.org/10.1029/97TC03140 fractured carbonate rocks: Insights from advanced cathodoluminescence Yin, X., Huang, W., Lu, S., Wang, P., Wang, W. and Xia, L. 2016. The imaging. Tectonophysics, 690, 190–205, https://doi.org/10.1016/j.tecto.2016. connectivity of reservoir sand bodies in the Liaoxi Sag, Bohai Bay Basin: 05.001 insights from three dimensional stratigraphic forward modeling. Marine and Vahrenkamp, V.C., David, F., Duijndam, P., Newall, M. and Crevello, P. 2004. Petroleum Geology, 77, 1081–1094, https://doi.org/10.1016/j.marpetgeo. Growth architecture, faulting, and karstification of a middle Miocene 2016.08.009 Downloaded from http://jgs.lyellcollection.org/ by guest on September 30, 2021

E. Ukar et al.

Yu, B.S., Lin, C.S., Fan, T.L., Wang, L.D., Gao, Z.Q. and Zhang, C. 2011. Arabian Journal of Geosciences, 9, 533, https://doi.org/10.1007/s12517-016- Sedimentary response to geodynamic reversion in Tarim Basin during 2557-9 Cambrian and Ordovician and its significance to reservoir development. Earth Zheng, X.P., Shen, A.J., Shou, J.F. and Pan, W.Q. 2009. A quantitative plate of Science Frontiers, 18, 221–232 [in Chinese with English abstract]. collapsed karst cave depth and its application in geological prediction and Zeng, H., Loucks, R., Janson, X., Wang, G., Xia, Y., Yuan, B. and Xu, L. 2011a. evaluation of carbonate reservoir. Marine Origin Petroleum Geology, 14, Three-dimensional seismic geomorphology and analysis of the Ordovician 55–59 [in Chinese with English abstract]. paleokarst drainage system in the central Tabei Uplift, northern Tarim Basin, Zou, C. 2012. Unconventional Petroleum Geology. Elsevier, Amsterdam. western China. AAPG Bulletin, 95, 2061–2083, https://doi.org/10.1306/ Zou, C., Tao, S. et al. 2009. Formation and distribution of “continuous” gas 03111110136 reservoirs and their giant gas province: A case study from the Upper Triassic Zeng, H., Wang, G., et al. 2011b. Characterizing seismic bright spots in deeply Xujiahe Formation giant gas province, Sichuan basin. Petroleum Exploration buried, Ordovician Paleokarst strata, Central Tabei uplift, Tarim Basin, and Development, 36, 307–319 [in Chinese with English abstract], https://doi. Western China. Geophysics, 76, B127-B137, https://doi.org/10.1190/1.3581199 org/10.1016/S1876-3804(09)60128-2 Zhang, C., Song, H., Chen, A. and Wu, Z. 1996. Mesozoic thrust tectonics in Zhu, G., Zhang, S. et al. 2012. The occurrence of ultra-deep heavy oils in the Yanshan intraplate orogenic belt. In: Wu, Z. and Chai, Y. (eds) Tectonics of Tabei Uplift of the Tarim Basin, NW China. Organic Geochemistry, 52, China: Proceedings of the 1995 Annual Conference of Tectonics in China. 88–102, https://doi.org/10.1016/j.orggeochem.2012.08.012 Geological Publishing House, Beijing, 77–82. Zhu, G., Zhang, S. et al. 2013a. A well-preserved 250 million-year-old oil Zhang, Q., Qin, F., Liang, B., Cao, J., Dan, Y., Li, J. and Chen, L. 2018. accumulation in the Tarim Basin, western China: Implications for hydrocar- Characteristics of Ordovician paleokarst inclusions and their implications for bon exploration in old and deep basins. Marine and Petroleum Geology, 43, paleoenviroumental and geological history in Halahatang area of northern 478–488, https://doi.org/10.1016/j.marpetgeo.2012.12.001 Tarim Basin. Carbonates and Evaporites, 33,43–54, https://doi.org/10.1007/ Zhu, G., Su, J. et al. 2013b. Formation mechanisms of secondary hydrocarbon s13146-016-0325-2 pools in the Triassic reservoirs in the northern Tarim Basin. Marine and Zhang, X.Y., Gu, J.Y., Luo, P., Zhu, R.K. and Luo, Z. 2006. Genesis of the Petroleum Geology, 46,51–66, https://doi.org/10.1016/j.marpetgeo.2013.06. fluorite in the Ordovician and its significance to the petroleum geology of 006 Tarim Basin. Acta Petrologica Sinica, 22, 2220–2228. Zhu, G., Wang, H. and Weng, N. 2016. TSR-altered oil with high-abundance Zhang, Y.Y. and Luo, X.Q. 2011. K-Ar dating authigenic illites and the hydrocarbon thiaadamantanes of a deep-buried Cambrian gas condensate reservoir in Tarim accumulation history of the Silurian bituminous sandstone reserviors in the Basin. Marine and Petroleum Geology, 69,1–12, https://doi.org/10.1016/j. Yingmaili area, Tarim Basin. Petroleum Exploration and Development, 38, marpetgeo.2015.10.007 203–210. Zhu, G., Liu, X., Yang, H., Su, J., Zhu, Y., Wang, Y. and Sun, C. 2017. Genesis Zhang, Y.Y., Zwingmann, H., Todd, A., Liu, K. and Lou, X. 2004. K-Ar dating of and distribution of hydrogen sulfide in deep heavy oil of the Halahatang area in authigenic illites and its applications to study of oil-gas charging histories of the Tarim Basin, China. Journal of Natural Gas Geoscience, 2,57–71, https:// typical sandstone reservoirs, Tarim Basin, Northwest China. Earth Science doi.org/10.1016/j.jnggs.2017.03.004 Frontiers, 11, 637–647. Zhu, G., Milkov, A.V. et al. 2018. Non-cracked oil in ultra-deep high-temperature Zhang, Z., Wu, S., Gao, Z. and Ziao, S.B. 1983. Research on sedimentary model reservoirs in the Tarim basin, China. Marine and Petroleum Geology, 89, from Late Carboniferous to Early Permian Epoch in Kalpin region, Xinjiang. 252–262, https://doi.org/10.1016/j.marpetgeo.2017.07.019 Xinjiang Geology, 1,9–20. [in Chinese with English abstract]. Zhu, G., Milkov, A.V. et al. 2019. Formation and preservation of a giant Zhao, W., Shen, A., Qiao, Z., Zheng, J. and Wang, X. 2014. Carbonate karst petroleum accumulation in superdeep carbonate reservoirs in the southern reservoirs of the Tarim Basin, northwest China: Types, features, origins, and Halahatang oil field area, Tarim Basin, China. AAPG Bulletin, 103, implications for hydrocarbon exploration. Interpretation, 2, SF65–SF90, 1703–1743, https://doi.org/10.1306/11211817132 https://doi.org/10.1190/INT-2013-0177.1 Zubovich, A.V., Wang, X. et al. 2010. GPS velocity field for the Tien Shan and Zhao, J., Lin, C., Liu, J., Yang, H. and Cai, Z. 2016. Sedimentary facies and surrounding regions. Tectonics, 29, TC6014, 1–23, https://doi.org/10.1029/ sequence stratigraphy of the Silurian at Tabei uplift, Tarim Basin, China. 2010TC002772