Structural Diagenesis in Carbonate Rocks As Identified in Fault

Article Structural Diagenesis in Carbonate Rocks as Identiﬁed in Fault Damage Zones in the Northern Tarim Basin, NW China

Guanghui Wu 1,2,*, En Xie 3, Yunfeng Zhang 1, Hairuo Qing 2, Xinsheng Luo 3 and Chong Sun 3 1 School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China; [email protected] 2 Department of Geology, University of Regina, Regina, SK S4S 0A2, Canada; [email protected] 3 PetroChina Tarim Oilﬁeld Company, Korla 841000, China; [email protected] (E.X.); [email protected] (X.L.); [email protected] (C.S.) * Correspondence: [email protected]

Received: 5 May 2019; Accepted: 22 May 2019; Published: 13 June 2019

Abstract: The identification of structural diagenesis and the reconstruction of diagenetic paragenesis in fault damage zones is important for understanding fault mechanisms and fluid flow in the subsurface. Based on the examination of core and sample thin section data, we deciphered the diagenetic parasequence and their fault controls for Ordovician carbonates in the northern Tarim intracratonic basin in NW China (Halahatang area). In contrast to the uniform nature of diagenesis observed in country rocks, there is a relatively complicated style of compaction and pressure solution, multiple fracturing, and cementation and dissolution history along the carbonate fault damage zones. The relative paragenetic sequence of the structure related diagenesis suggests three cycles of fracture activities, following varied fracture enlargement and dissolution, and progressively weaker calcite cementation. These processes of structure related diagenesis are constrained to the fault damage zones, and their variation is affected by the fault activities. The results of this study suggest that the carbonate reservoir and productivity could be impacted by the structure related diagenesis locally along the fault damage zones.

Keywords: fault damage zone; carbonate; fracture; structural diagenesis; diagenetic sequence

1. Introduction Brittle deformation (e.g., faulting) in the upper crust is a common phenomenon that can strongly influence any subsequent geological processes and indeed the hydraulic properties of the rocks affected by this type of deformation [1–3]. The generally complicated deformation structures, typical of large faults [1–6], may impart a complex diagenesis to these faulted zones [7–9]. For example, structures related to diagenesis can be pervasive and may alter mineralization pathways and cementation in a fault zone, especially in the case of carbonate rocks [8–10]. While the structure and microstructure of fault zones have been widely studied to better understand the process of faulting, by contrast, the diagenesis of and its integration with fault processes have been mostly neglected [1,2,9]. Structural diagenesis is the study of the relationships between deformation or deformational structures and chemical changes to sediments [9]. Whereas fault architecture and process can constrain the diagenesis along a fault zone, diagenetic processes can significantly influence the fault architecture, rock properties and mechanisms of brittle deformation leading to fracture or faulting [9–15]. Studies on the integration of fault process and structural diagenesis are helpful to understand not only the mechanisms of faulting, but also petrophysical properties and fluid flow along fault zones, promoting efforts in multidisciplinary studies of structure, reservoir characterization and diagenesis [9,16–22]. However, the fault elements and

Minerals 2019, 9, 360; doi:10.3390/min9060360 www.mdpi.com/journal/minerals Minerals 2019, 9, 360 2 of 15 Minerals 2018, 8, x FOR PEER REVIEW 2 of 15 diagenesisinteraction and are their demonstrably interaction arecomplicated demonstrably and complicatedvaried in fault and zones, varied particularly in fault zones, in the particularly presence of inmulti-stage the presence structural of multi-stage and diagenetic structural processes and diagenetic [15,19,23,24]. processes [15,19,23,24]. InIn this this paper, paper, we we present present data data from from and and discuss discus as carbonatea carbonate fault fault system system with with three three distinct distinct phasesphases of faultof fault activity activity in the in intracratonic the intracratonic Tarim Basin,Tarim China.Basin, GeologicalChina. Geological rock cores rock and cores thin sections and thin weresections studied were and studied used to and determine used to determine the fault-related the fault-related diagenesis diagenesis of the carbonate of the carbonate “damage zones”.“damage Moreover,zones”. Moreover, we examine we the examine fracture the cross-cutting fracture cross-cutting and abutting and relationships abutting relationships and orientations and orientations of veins to understandof veins to understand the parasequence the parasequence of the structural of the diagenesis. structural In diagenesis. addition, we In discuss addition, the inﬂuenceswe discuss of the faultinfluences activities of on fault the activities carbonate on diagenesis the carbonate in the diagenesis deep subsurface. in the deep subsurface.

2. Geological2. Geological Setting Setting 2 TheThe Tarim Tarim Basin Basin is theis the largest largest petroliferous petroliferous basin basi inn NWin NW China, China, with with an an area area of 560,000of 560,000 km km[252 [25]] (Figure(Figure1a). 1a). It hasIt has an an Archean-Early Archean-Early Neoproterozoic Neoproterozoic crystalline crystalline basement basement that that is coveredis covered by by thick thick sequencesequence of lateof Neoproterozoic-Quaternary late Neoproterozoic-Quatern sedimentaryary stratasedimentary with multi-stage strata sedimentary-tectonic with multi-stage evolutionsedimentary-tectonic (Figure1b) [ 25 evolution]. Key events (Figure include 1b) [25]. supercontinent Key events include breakup supercontinent in the late Neoproterozoic, breakup in the thelate opening Neoproterozoic, and closure the of opening Tethys in and the closure Palaeozoic of Te andthys the in Mesozoic,the Palaeozoic and theand Indo-Asian the Mesozoic, collision and the duringIndo-Asian the Cenozoic collision [25 during–27]. Thethe Cenozoic Tarim Basin [25–2 developed7]. The Tarim from Basin the Cenozoic developed foreland from the basin Cenozoic and theforeland Palaeozoic-Mesozoic basin and the intracratonicPalaeozoic-Mesozoic basin [25 intracratonic], following multi-phase basin [25], following tectonic movements, multi-phase which tectonic involvedmovements, various which faults involved [25,27–30 various], developed faults in[25,27–30], the intracratonic developed basin. in the intracratonic basin.

Figure 1. (a) The tectonic division of the Tarim Basin (the lower right inset map shows its location inFigure China) 1. (after (a) The [30 tectonic]); (b) Geological division of proﬁlethe Tarim across Basin the (the study lower area right (the inset upper map strata shows of its ~3000 location m in Cretaceous-QuaternaryChina) (after [30]); are(b) omittedGeological in the profile cross-section; across the modiﬁed study fromarea [ 30(the]). єupper: Cambrian; strata O of3l: Upper~3000 m

OrdovicianCretaceous-Quaternary Lianglitage Formation; are omitted O3s: in Upper the cross-section; Ordovician Samgtamu modified Formation;from [30]). S:є: Silurian;Cambrian; C-P: O3l: Carboniferous-Permian;Upper Ordovician Lianglitage T: Triassic; Formation; J: Jurassic; O K:3s: Cretaceous; Upper Ordovician E-Q: Eocene-Quaternary. Samgtamu Formation; S: Silurian; C-P: Carboniferous-Permian; T: Triassic; J: Jurassic; K: Cretaceous; E-Q: Eocene-Quaternary.

The Halahatang area investigated in this study, covering about 8000 km2, is located in the southern slope of the Northern Uplift, Tarim Basin (Figure 1a). At this site, the Phanerozoic strata

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The Halahatang area investigated in this study, covering about 8000 km2, is located in the southern Mineralsslope of 2018 the, 8 Northern, x FOR PEER Uplift, REVIEW Tarim Basin (Figure1a). At this site, the Phanerozoic strata are relatively3 of 15 complete, consisting of Cambrian-Ordovician carbonates, with a Silurian to Cretaceous sequence are relatively complete, consisting of Cambrian-Ordovician carbonates, with a Silurian to Cretaceous containing multiple unconformities, and a Cenozoic sequence representing rapid subsidence [25,29] sequence containing multiple unconformities, and a Cenozoic sequence representing rapid (Figures1b,2a and3a). Regional structural interpretation and fault mapping reveals a network of subsidence [25,29] (Figures 1b, 2a and 3a). Regional structural interpretation and fault mapping conjugate strike-slip fault systems that developed in the Cambrian–Ordovician, followed by some reveals a network of conjugate strike-slip fault systems that developed in the Cambrian–Ordovician, reactivation, in Silurian–Permian and Mesozoic-Eocene times by the process of faulting inheritance, followed by some reactivation, in Silurian–Permian and Mesozoic-Eocene times by the process of with multi-layer ﬂower structures (Figure2)[ 29,30]. Many faults terminate at the top of the Upper faulting inheritance, with multi-layer flower structures (Figure 2) [29,30]. Many faults terminate at Ordovician carbonate rocks; these generally show positive ﬂower structures in Lower Palaeozoic the top of the Upper Ordovician carbonate rocks; these generally show positive flower structures in strata, which resulted in horst blocks in the overlapping zones (Figure2a) [ 29]. Fault zones in the Lower Palaeozoic strata, which resulted in horst blocks in the overlapping zones (Figure 2a) [29]. Ordovician carbonates are up to 50–70 km in length (Figure2b), but the vertical throw along a fault Fault zones in the Ordovician carbonates are up to 50–70 km in length (Figure 2b), but the vertical zone is generally in a range of 20–80 m [30]. throw along a fault zone is generally in a range of 20–80 m [30].

Figure 2. (a) Typical seismic sections showing the strike-slip faults (the strata symbols are same with Figure 2. (a) Typical seismic sections showing the strike-slip faults (the strata symbols are same with Figure1), and ( b) the map view of the strike-slip fault system affecting the Ordovician carbonate Figure 1), and (b) the map view of the strike-slip fault system affecting the Ordovician carbonate sequence (~6500–7500 m) in the Halahatang area, Tarim Basin (Modified from [30]). sequence (~6500–7500 m) in the Halahatang area, Tarim Basin (Modified from [30]). On the basis of seismic responses analysis, the integrated application of coherent enhancement technique,On the Automaticbasis of seismic Fault responses Extraction analysis, (AFE, Figure the integrated3b) and other application seismic of attributes coherent areenhancement useful for technique,carbonate faultAutomatic “damage Fault zone” Extraction identification (AFE, under Figure the 3b) deep and subsurface other seismic [31]. Theattributes result ofare this useful analysis for carbonateshowed that fault carbonate “damage fault zone” damage identification zones are widely under distributedthe deep subsurface as fan-shape [31]. features The alongresult theof faultthis analysiszone, generally showed with that a carbonate width of 200~2500 fault damage m (Figure zone3sb) are [ 30 widely,31]. These distributed fault damage as fan-shape zones have features good alongagreement the fault with zone, the high generally well production with a width within of 200~2500 1200 m of m the (Figure fault cores 3b) [30,31]. [30]. In These more thanfault 200 damage wells, zonesthat penetrated have good the agreement Ordovician with carbonates, the high well almost prod alluction wells attainedwithin 1200 a high m productivityof the fault cores fall into [30]. fault In moredamage than zones, 200 althoughwells, that some penetrated of the wells the Ordovician still yield a lowcarbonates, productivity almost in theall damagewells attained zones [a30 high,31]. productivity fall into fault damage zones, although some of the wells still yield a low productivity in the damage zones [30,31].

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Figure 3. a Figure (3.) ( Stratigraphica) Stratigraphic column column of of the the Ordovician Ordovician carbonate carbonate in in the the Halahatang Halahatang area, area, China,China, andand (b) (b) Automatic Fault Extraction (AFE) description of the damage zones (yellow areas) affecting the Automatic Fault Extraction (AFE) description of the damage zones (yellow areas) affecting the Ordovician carbonate (after [31]) in this area (the black dots show well locations). Ordovician carbonate (after [31]) in this area (the black dots show well locations). In the Halahatang study area, the top of the Ordovician was buried at a depth of 6500–7500 m and In the Halahatang study area, the top of the Ordovician was buried at a depth of 6500–7500 m dips gently towards the south of the basin (Figure1b) [ 28]. The primary reservoirs of the Yjianfang and dips gently towards the south of the basin (Figure 1b) [28]. The primary reservoirs of the Formation are mainly reef-shoal facies in a broad ramp platform [28,32], with a thickness of up to Yjianfang Formation are mainly reef-shoal facies in a broad ramp platform [28,32], with a thickness 200 m (Figure3a). The bulk of the carbonate reservoirs are rich in grainstones, formed from the large of up to 200 m (Figure 3a). The bulk of the carbonate reservoirs are rich in grainstones, formed from shoal facies interbedded with small reefs [28]. There are large areas of tight carbonates in the broad the large shoal facies interbedded with small reefs [28]. There are large areas of tight carbonates in slope that has a very low matrix porosity (<5%), and low permeability (<1 mD), with considerable the broad slope that has a very low matrix porosity (<5%), and low permeability (<1 mD), with heterogeneity [28]. The major production in this area is from dissolution vugs (diameter between considerable heterogeneity [28]. The major production in this area is from dissolution vugs 2–100 mm) and caves (diameter >100 mm) found in the tight matrix carbonate reservoirs [28,33]. (diameter between 2–100 mm) and caves (diameter >100 mm) found in the tight matrix carbonate The "sweet spots" of fracture-cave reservoirs (assemblages of caves, vugs and fractures) showing reservoirs [28,33]. The "sweet spots" of fracture-cave reservoirs (assemblages of caves, vugs and “bead-shape” strong reflection in the seismic profile [28]) are main targets for the production in the fractures) showing “bead-shape” strong reflection in the seismic profile [28]) are main targets for the carbonate reservoirs [29,33,34]. The fracture-cave reservoirs are quite different from the high matrix production in the carbonate reservoirs [29,33,34]. The fracture-cave reservoirs are quite different reservoirs and meteoric karst reservoirs [35,36], but are strongly heterogeneous reservoirs that are close from the high matrix reservoirs and meteoric karst reservoirs [35,36], but are strongly to hypogene or hydrothermal karstification along fault zones [37–39]. heterogeneous reservoirs that are close to hypogene or hydrothermal karstification along fault 3. Datazones and [37–39]. Methods

3. CoresData and were Methods available for study from more than 20 wells penetrating the Halahatang area Ordovician carbonates along the fault damage zones. These enable core and thin section data to be used in describing the microstructuresCores were andavailable structure for relatedstudy from diagenesis more inthan the 20 carbonate wells penetrating fault zones. the We Halahatang analyzed the area sequenceOrdovician of the carbonates fractures by along examining the fault cross-cutting damage zones. and termination These enable relationships, core and thin ﬁlling section and cementdata to be generations,used in describing and orientation the microstructures in cores and and thin structure section related samples. diagenesis Pink-dye in resin the carbonate impregnated fault thinzones. sectionsWe analyzed were used the tosequence identify of porosity. the fractures FMI by (micro-resistivity examining cross-cutting image logging) and termination allowed for relationships, fracture orientationfilling and analysis cement [30 generations,]. These measurements and orientation were usedin cores to discuss and thin the fracturesection samples. paragenetic Pink-dye sequence. resin Togetherimpregnated with the thin timing sections constrains were by used fault to activities, identify weporosity. present FMI a relatively (micro-resistivity paragenetic image sequences logging) of structuralallowed diagenesis for fracture in orientation the carbonate analysis fault damage [30]. Thes zones.e measurements In contrast to were the diagenesisused to discuss in the the country fracture rocks,paragenetic we discuss sequence. the faulting Together impacts with on the the timing diagenesis constrains along theby carbonatefault activities, fault damagewe present zones a relatively in the Tarimparagenetic Basin. sequences of structural diagenesis in the carbonate fault damage zones. In contrast to the diagenesis in the country rocks, we discuss the faulting impacts on the diagenesis along the carbonate fault damage zones in the Tarim Basin.

4. Structure Related Diagenesis

4.1. Compaction and Pressure Solution

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4. Structure Related Diagenesis

4.1.Minerals Compaction 2018, 8, x andFOR PressurePEER REVIEW Solution 5 of 15

TheThe OrdovicianOrdovician countrycountry rocks,rocks, inin the the deep deep subsurface subsurface ( >(>60006000 m),m), exhibitexhibit evidenceevidence ofofstrong strong compactioncompaction with with line line and and edge edge contacts contacts among among the mineralthe mineral grains grains in the carbonatesin the carbonates (Figure 4(Figurea) [ 28,40 4a)]. In[28,40]. the fault In damagethe fault zones, damage some zones, thin sectionssome thin show sections ﬂoating show and floating point contacts and point of thecontacts grainstones of the (Figuregrainstones4b), and (Figure point 4b), and and line contactspoint and in line other contacts grainstones in other (Figure grainstones4c). The variable(Figure 4c). compaction The variable in thecompaction carbonate in fault the damage carbonate zones fault is inconsistent damage zones with theis inconsistent relatively strong with and the uniform relatively compaction strong and in theuniform host rocks. compaction In this context,in the host the faultrocks. activity In this possibly context, hadthe anfault impact activity on thepossibly carbonate had diagenesisan impact inon thethe fault carbonate damage diagenesis zones. in the fault damage zones.

FigureFigure 4.4. PhotomicrographsPhotomicrographs showingshowing compactioncompaction observedobserved inin thethe thinthin sectionssections (pink-dye(pink-dye resinresin impregnatedimpregnated to to better better illustrate illustrate sample sample porosity). porosity). (a)Grainstone (a) Grainstone showing showing line contacts line contacts in country in country rocks, H6-1,rocks, 6667.93 H6-1, m;6667.93 (b) floating m; (b grains) floating in grainstone grains in showinggrainstone weak showing compaction weak incompaction a fault damage in a zone, fault H7-3,damage 6644.47 zone, m; H7-3, (c) strong 6644.47 compaction m; (c) strong and recrystallizationcompaction and inrecrystal a faultlization damage in zone, a fault H803, damage 6578 m.zone, H803, 6578 m. Pressure solution is evident by the common occurrence of stylolites [41,42] in the Halahatang area OrdovicianPressure solution carbonate is evident rocks (Figure by the5 ).common Bedding-parallel occurrence stylolites of stylolites occur [41,42] throughout in the the Halahatang studied successionarea Ordovician of carbonates. carbonate The rocks stylolites (Figure exhibit 5). Bedding-parallel different morphologies. stylolites Many occur bedding-parallel throughout the stylolites studied (S1-1)succession were observed of carbonates. in core withThe lowstylolites amplitude, exhibi wave-liket different profiles. morphologies Some stylolites. Many (S1-2) bedding-parallel have higher amplitudesstylolites (S1-1) forms, were rectangular observed layer in core type with and low seismogram amplitude, pinning wave-like type, profiles. and low Some intersection stylolites angles (S1-2) withhave the higher strata amplitudes and bedding-parallel forms, rectangular stylolites (S1-1)layer (Figuretype and5a,b). seismogram These stylolites pinning (S1-2) type, merge and and low overprintintersection those angles of the with bedding-parallel the strata and stylolites bedding-p (S1-1),arallel suggesting stylolites a postdate (S1-1) pressure (Figure solution5a,b). These [43]. Stylolitesstylolites were (S1-2) encountered merge and mainly overprint in wackstone-packstone those of the bedding-parallel rather than instylolites sparry cement(S1-1), suggesting grainstones, a whichpostdate suggests pressure a lithological solution [43]. control Stylolites on pressure were encountered solution. Some mainly stylolites in wackstone-packstone (S2) are perpendicular rather or obliquethan in tosparry bedding cement (Figure grainstones,5b,c). These which stylolites suggests are a generallylithological classified control ason tectonicpressure stylolites solution.with Some surfacesstylolites perpendicular (S2) are perpen todicular the largest or oblique compressive to bedding principal (Figure stress 5b,c). axis These [43]. Thestylolites tectonic are stylolitesgenerally areclassified generally as sub-perpendiculartectonic stylolites to,with terminate surfaces or perpendicular intersect with theto the bedding-parallel largest compressive stylolites. principal Most tectonicstress axis stylolites [43]. The have tectonic low-amplitudes stylolites are and generally sharp peaks.sub-perpendicular They are typically to, terminate enlarged or intersect and partial with filledthe bedding-parallel with calcite cements stylolites. and arenopelitic Most tectonic fillings. stylol Inites the have country low-amplitudes rocks, there and is generally sharp peaks. one set They of bedding-parallelare typically enlarged stylolites and with partial serrate filled or with wave-like calcite profilescements within and arenopelitic cores (Figure fillings.5d). These In the features country suggestrocks, there a complicated is generally diagenesis one set for of the bedding-parall observed stylolitesel stylolites in the with fault serrate damage or zones wave-like in contrast profiles to thewithin tight cores stylolites (Figure in the 5d). host These carbonate features rocks. suggest a complicated diagenesis for the observed stylolites in the fault damage zones in contrast to the tight stylolites in the host carbonate rocks.

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The Ordovician country rocks, in the deep subsurface (>6000 m), exhibit evidence of strong compaction with line and edge contacts among the mineral grains in the carbonates (Figure 4a) [28,40]. In the fault damage zones, some thin sections show floating and point contacts of the grainstones (Figure 4b), and point and line contacts in other grainstones (Figure 4c). The variable compaction in the carbonate fault damage zones is inconsistent with the relatively strong and uniform compaction in the host rocks. In this context, the fault activity possibly had an impact on the carbonate diagenesis in the fault damage zones.

Figure 4. Photomicrographs showing compaction observed in the thin sections (pink-dye resin impregnated to better illustrate sample porosity). (a) Grainstone showing line contacts in country rocks, H6-1, 6667.93 m; (b) floating grains in grainstone showing weak compaction in a fault damage zone, H7-3, 6644.47 m; (c) strong compaction and recrystallization in a fault damage zone, H803, 6578 m.

Pressure solution is evident by the common occurrence of stylolites [41,42] in the Halahatang area Ordovician carbonate rocks (Figure 5). Bedding-parallel stylolites occur throughout the studied succession of carbonates. The stylolites exhibit different morphologies. Many bedding-parallel stylolites (S1-1) were observed in core with low amplitude, wave-like profiles. Some stylolites (S1-2) have higher amplitudes forms, rectangular layer type and seismogram pinning type, and low intersection angles with the strata and bedding-parallel stylolites (S1-1) (Figure 5a,b). These stylolites (S1-2) merge and overprint those of the bedding-parallel stylolites (S1-1), suggesting a postdate pressure solution [43]. Stylolites were encountered mainly in wackstone-packstone rather than in sparry cement grainstones, which suggests a lithological control on pressure solution. Some stylolites (S2) are perpendicular or oblique to bedding (Figure 5b,c). These stylolites are generally classified as tectonic stylolites with surfaces perpendicular to the largest compressive principal stress axis [43]. The tectonic stylolites are generally sub-perpendicular to, terminate or intersect with the bedding-parallel stylolites. Most tectonic stylolites have low-amplitudes and sharp peaks. They are typically enlarged and partial filled with calcite cements and arenopelitic fillings. In the country rocks, there is generally one set of bedding-parallel stylolites with serrate or wave-like profiles within cores (Figure 5d). These features suggest a complicated diagenesis for the observed stylolites Minerals 2019, 9, 360 6 of 15 in the fault damage zones in contrast to the tight stylolites in the host carbonate rocks.

Figure 5. Photographs showing stylolites in cores. (a) Bedding-parallel, low-amplitude stylolite (S1-1) and high-amplitude stylolite (S1-2), H601-1, 6633.28 m; (b) oblique stylolite (S1-2) merged into bedding-parallel stylolite (S1-1), tectonic stylolite (S2) terminated in the oblique stylolite (S1-2), H902, 6641.61 m; (c) tectonic stylolite (S2) cross-cutting S1-1 stylolites and terminated in S1-2 stylolites; dissolution and enlargement along S1-2 and S2 stylolites in a fault damage zone, R4, 6625 m; (d) bedding-parallel stylolites (S1-1) in mudstone in country rocks, H6, 7044.21 m.

4.2. Cementation The primary porosity of the Halahantang area Ordovician carbonate rocks is almost completely occluded (Figures4 and6a) [ 28,40]. The secondary porosity in fractures and vugs is also filled mainly by calcite cements and some bitumen (Figure6). Thin section analysis reveals that the early sparry cements generally hindered further compaction (Figures4 and6a), suggesting that compaction and cementation were synchronous with early shallow burial depths. The country rocks are characterized by fine-grained, thin, equidimensional cements (C1-1) surrounding the grains, and coarser, equant, granular cements (C1-2) that infilled in the intergranular porosity (Figure6a). The fine cements coating the grains are assumed to have occurred in a marine diagenetic environment [28,40]. As such, there were possibly multiple cementation generations affecting the intergranular porosity of the Ordovician carbonate in the Tarim Basin [40], but herein we only discuss those that are related to fault activity. In the fault damage zones, there were generally observed 2-4 generations of calcite cement [28] (Figure6). The intergranular cements (C1-2) are cut by later fractures (Figure6b) [ 28,30], suggesting local fault related cements occurred after the eogenetic cementation (C1). Two stages of cement were observed within the fractures (Figure6c,e) from the fault zones, as well as in the vugs [ 28]. Veins within fractures are generally filled with coarse blocky cements (C2) towards their center, and finer, calcite cements (C3) along fracture walls (Figure6c), suggesting the fracture enlarged and precipitating the coarse calcite cements in the center, post crystallization of the narrow, finer calcite along the fracture wall. In the sample thin sections (Figure6d), a late vein (C3) terminated an earlier generation of veins (C2) possibly indicating a late stage of cementation. In addition, bitumen impregnated cements (C3) divided by early white cements (C2) in the fracture center also favor at least two stages of vein cementation (Figure6e). These early vein cements (D2) are generally wider than the late cements (D3), consistent with a stronger fault activity in the early stage [29,30]. The early vein cements are generally fully infilled and are cross-cut or overprinted by the latest fractures (Figure6f), although the latest fractures have little by way of cement (D4). These observations attest to at least three generations of pore/fracture cementation (C2-C4), following on from the main stage of compaction and cementation (C1) during eogenetic environment. Minerals 2018, 8, x FOR PEER REVIEW 6 of 15

FigureFigure 6.6. Photographs and photomicrographs illustrating cementation andand dissolutiondissolution texturestextures observedobserved inin corecore ((ee)) andand thinthin sectionsection samplessamples ((aa–d,f–d,f)) ofof thethe OrdovicianOrdovician carbonatescarbonates inin thethe HalahatangHalahatang areaarea (pink-dye(pink-dye resinresin impregnatedimpregnated toto betterbetter showshow fracturefracture porosity).porosity). (a)) FineFine cementscements (C1-1)(C1-1) withwith isopachousisopachous coatingcoating the the grains, grains, coarse coarse granular granular cements cements (C1-2) (C1-2) within within the the intergranular intergranular porosity porosity in the in countrythe country rocks, rocks, X101, 6822.08X101, 6822.08 m; (b) the m; late (b) fracture the late filled fracture cements filled (C2) cements cut across (C2) and overprintedcut across and the earlyoverprinted intergranular the early cements intergranular (C1), H7-1, cements 6579.58 (C1), m; (c )H7-1, two stages 6579.58 of cementationm; (c) two stages filling inof acementation micro-fault, withfilling earlier in a widemicro-fault, cements with (C2) inearlier the center wide separatedcements the(C2) narrow in the finecenter cements separated on the the wall narrow (C3), H803, fine 6598.85cements m; on (d the) a narrowerwall (C3), cement H803, vein6598.85 (C3) m; terminated (d) a narrower on a wider cement vein vein (C2), (C3) H11-1, terminated 6683 m; on (e) a whitewider cementvein (C2), (C2) H11-1, occurring 6683 in them; fracture(e) a white center cement divides (C2) the bitumenoccurring impregnated in the fracture cements center (C3) todivides highlight the twobitumen stages impregnated of cementation, cements H801, (C3) 6733.6 to highlight m; (f) late two partially stages of filled cementation, cements (C4) H801, in the6733.6 fracture m; (f) cuts late acrosspartially and filled merged cements with the(C4) earlier in the fully fracture cemented cuts across (C3) fracture, and merged H7-1, with 6581.22 the m.earlier fully cemented (C3) fracture, H7-1, 6581.22 m. 4.3. Fracturing MultipleIn the fault fractures damage developed zones, there upon were a wide generally fault damage observed zone 2-4 with generations a width ofof morecalcite than cement 2 km, [28] in the(Figure carbonate 6). The rocks intergranular in Halahatang cements area [(C1-2)30,31]. are The cut fractures by later mainly fractures have (Figure steep dips 6b) ([28,30],> 600) as suggesting observed inlocal core fault and related FMI images cements [29 ,occurred30] (Figure after7a–c). the Analysiseogenetic of cementation thin sections (C1). and Two cores stages (Figure of7 cementd–f) shows were thatobserved there arewithin generally the fractures two sets (Figure of fractures 6c,e) developedfrom the fault in the zones, Ordovician as well carbonate as in the rocks. vugs FMI[28]. imageVeins analysis also revealed two sets of fractures in this area [30], striking NE and NW, which is subparallel to the strike-slip faults throughout the study area (Figure2b). Some of the conjugate fractures and en echelon fractures are similar to those of the fault system [30], suggesting the self-similarity between the fault system and its microstructures. In general, only one set of fractures developed preferentially in a well. Most fractures have narrow aperture (Figure7a); some show wider aperture, particularly in multiple reactivated fractures (F1 in Figure7b,c). The fracture apertures generally range from 0.1 mm to 5 mm in core, and 0.005 to 0.1 mm in sample thin sections. A few veins with aperture values of more than 1 cm were observed in core. The fractures are characterized by multiple enlargements and almost all are completely infilled by calcite cement(s) (F1 in Figure7a–c), and arenopelitic fillings and bitumen (F2 in Figure7b,c). A few fractures are partially calcite-filled and host bitumen. Minerals 2019, 9, 360 8 of 15 Minerals 2018, 8, x FOR PEER REVIEW 8 of 15

FigureFigure 7.7. PhotographsPhotographs and photomicrographsphotomicrographs showing fractures in thethe corescores ((aa–c–c)) andand thinthin sectionssections ((dd–f–f)) ofof the the Ordovician Ordovician carbonates carbonates in in the the Halahatang Halahatang area. area. (a) One(a) One vertical vertical fracture fracture (F1-1) (F1-1) and oneand high one anglehigh angle fracture fracture (F1-2) (F1-2) showing showing a narrow a narrow aperture aperture and cements and cements filling, filling, H902, H902, 6632.12 6632.12 m; (b) m; high (b) angle high fracturesangle fractures infilled infilled by calcite by cementscalcite cements (F1), reactivated (F1), reactivated open fracture open fracture filled with filled bitumen with bitumen (F2) and late(F2) subparalleland late subparallel fractures atfractures the tip (F3),at the H601-1, tip (F3), 6634.40 H601-1, m; 6634.40 (c) an early m; ( cementedc) an early fracture cemented (F1) fracture reactivated (F1) (F2)reactivated along the (F2) fracture along wallthe fractu and splitsre wall the and argillaceous splits the and argillaceous pyrite cements, and pyrite X101, cements, 6767.52 m; X101, (d) an6767.52 early wider,m; (d) enlargedan early fracturewider, enlarged (F1) terminated fracture a (F1) late narrowerterminated fracture a late (F2),narrower H11-1, fracture 6682.5 m;(F2), (e) H11-1, cross-cutting 6682.5 relationshipm; (e) cross-cutting showing relationship early wide fractureshowing (F1) early terminated wide fracture the second (F1) terminated generation the fracture second (F2), generation and cut acrossfracture by (F2), the latestand cut open across fracture by the (F3), late J7,st 7108.6open fracture m; (f) late (F3), open J7, 7108.6 fracture m; (F3) (f) cutslate open across fracture the former (F3) earlier,cuts across fully the infilled former fracture earlier, (F1), fully X7, infilled 6918.12 frac m.ture (F1), X7, 6918.12 m.

4.4. DissolutionFracture cross-cutting relationships and the cementation sequence in the core and sample thin sections suggest there were multiple stages of fracturing (Figures6d–f and7). The fracture enlargement The primary porosity of the Ordovician carbonate rocks from the Halahatang area has been and fillings show at least two stages of fracturing in the fault damage zones (Figures6c and7b,c). The almost totally occluded during burial diagenesis [28,40]. Reservoir porosity is primarily the result of first fracturing episode (F1) was almost all completely infilled by coarse calcite cement. A second dissolution [28,30,33,34]. Multiple dissolution events have affected the carbonate rocks in the fault stage of fracturing (F2) led to the enlargement of early fractures (F1) and initial cement cements (C2) damage zones whereby the dissolution porosity occurs along the fractures (Figure 8a), along (Figure6c) with those of a narrower aperture and finer cement. Some of the second fractures (F2) have rectangular stylolites (Figure 8b), and within the cataclasites (Figure 8c). Although many been distorted (Figure7d and terminated (Figure7e) by the earlier fractures (F1), and /or overprinted bedding-parallel stylolites (S1-1) were completely infilled to be effective barriers to fluid flow, some (Figure6c,e and Figure7b) in the earlier fractures (F1). The first stage fractures (F1) generally have stylolites have been reactivated to create some porosity and permeability in these rocks (Figures 5b wider aperture than the second stage fractures (F2) (Figure7b–e), consistent with the wider early and 8b). Note that much more dissolution vugs and pores are associated with tectonic stylolites cements (C2) (Figure6c,d). These fractures are almost completely filled with calcite cement. In contrast, rather than the sub-parallel stylolites (Figure 5c), suggesting a stronger reactivation and dissolution most of the late stage fractures (F3) that cross cut F1 and F2 fractures are open with fewer fillings of the tectonic stylolites. (Figure7e,f). The late fractures (F3) have somewhat enlarged earlier cemented fractures (Figure7b). The vein cementation is closely related to dissolution (Figure 6b,d), which likely provided 4.4.saturated Dissolution waters with calcite for cementation [44]. It is noted that the reactivated fractures are closely related to dissolution features, formed during fracture enlargement (Figures 7b and 8a). The primary porosity of the Ordovician carbonate rocks from the Halahatang area has been Thus, the wider aperture was generally furnished by the dissolution episode. Pervasive cementation almost totally occluded during burial diagenesis [28,40]. Reservoir porosity is primarily the result of occurred during F1 and F2 fracturing, as indicated by dissolution (Figures 6 and 7). Unfortunately, dissolution [28,30,33,34]. Multiple dissolution events have affected the carbonate rocks in the fault residual porosity is still poor as a result of the intense cementation during F1 and F2 fracturing, damage zones whereby the dissolution porosity occurs along the fractures (Figure8a), along rectangular particularly the case in the wider fractures near fault cores. There is generally weaker dissolution stylolites (Figure8b), and within the cataclasites (Figure8c). Although many bedding-parallel stylolites associated with the F3 fractures and accordingly little cementation along these late fractures (Figure (S1-1) were completely infilled to be effective barriers to fluid flow, some stylolites have been reactivated 7e,f). to create some porosity and permeability in these rocks (Figures5b and8b). Note that much more dissolution vugs and pores are associated with tectonic stylolites rather than the sub-parallel stylolites (Figure5c), suggesting a stronger reactivation and dissolution of the tectonic stylolites.

Minerals 2018, 8, x FOR PEER REVIEW 8 of 15

Figure 7. Photographs and photomicrographs showing fractures in the cores (a–c) and thin sections (d–f) of the Ordovician carbonates in the Halahatang area. (a) One vertical fracture (F1-1) and one high angle fracture (F1-2) showing a narrow aperture and cements filling, H902, 6632.12 m; (b) high angle fractures infilled by calcite cements (F1), reactivated open fracture filled with bitumen (F2) and late subparallel fractures at the tip (F3), H601-1, 6634.40 m; (c) an early cemented fracture (F1) reactivated (F2) along the fracture wall and splits the argillaceous and pyrite cements, X101, 6767.52 m; (d) an early wider, enlarged fracture (F1) terminated a late narrower fracture (F2), H11-1, 6682.5 m; (e) cross-cutting relationship showing early wide fracture (F1) terminated the second generation fracture (F2), and cut across by the latest open fracture (F3), J7, 7108.6 m; (f) late open fracture (F3) cuts across the former earlier, fully infilled fracture (F1), X7, 6918.12 m.

4.4. Dissolution The primary porosity of the Ordovician carbonate rocks from the Halahatang area has been almost totally occluded during burial diagenesis [28,40]. Reservoir porosity is primarily the result of dissolution [28,30,33,34]. Multiple dissolution events have affected the carbonate rocks in the fault damage zones whereby the dissolution porosity occurs along the fractures (Figure 8a), along rectangular stylolites (Figure 8b), and within the cataclasites (Figure 8c). Although many bedding-parallel stylolites (S1-1) were completely infilled to be effective barriers to fluid flow, some stylolites have been reactivated to create some porosity and permeability in these rocks (Figures 5b and 8b). Note that much more dissolution vugs and pores are associated with tectonic stylolites rather than the sub-parallel stylolites (Figure 5c), suggesting a stronger reactivation and dissolution of the tectonic stylolites. The vein cementation is closely related to dissolution (Figure 6b,d), which likely provided saturated waters with calcite for cementation [44]. It is noted that the reactivated fractures are closely related to dissolution features, formed during fracture enlargement (Figures 7b and 8a). Thus, the wider aperture was generally furnished by the dissolution episode. Pervasive cementation occurred during F1 and F2 fracturing, as indicated by dissolution (Figures 6 and 7). Unfortunately, residual porosity is still poor as a result of the intense cementation during F1 and F2 fracturing, particularly the case in the wider fractures near fault cores. There is generally weaker dissolution associatedMinerals 2019 ,with9, 360 the F3 fractures and accordingly little cementation along these late fractures (Figure9 of 15 7e,f).

Figure 8. Photographs showing dissolution in the carbonate fault damage zones in the Halahatang area (the pink-dye resin showing the porosity). (a) Multiple dissolution in cores, the ﬁrst dissolution (D1) porosity is occluded by calcite cements, the second dissolution (D2) porosity and vugs are ﬁlled with cements and bitumen, the late fracture (under arrow of D2) reactivation and opening possibly suggest third stage dissolution; H601-1, core, 6638 m; (b) dissolution porosity along a stylolite, H601-18, thin section, 6755.04 m; (c) dissolution porosity among the cataclasites, H803, thin section, 6573.78 m.

The vein cementation is closely related to dissolution (Figure6b,d), which likely provided saturated waters with calcite for cementation [44]. It is noted that the reactivated fractures are closely related to dissolution features, formed during fracture enlargement (Figures7b and8a). Thus, the wider aperture was generally furnished by the dissolution episode. Pervasive cementation occurred during F1 and F2 fracturing, as indicated by dissolution (Figures6 and7). Unfortunately, residual porosity is still poor as a result of the intense cementation during F1 and F2 fracturing, particularly the case in the wider fractures near fault cores. There is generally weaker dissolution associated with the F3 fractures and accordingly little cementation along these late fractures (Figure7e,f).

5. Discussion

5.1. Paragenetic Sequence of the Structure Related Diagenesis The relative paragenetic sequences of diagenesis are possibly inferred from fracture cross-cutting and abutting relationships and orientations, and the observed cementation relationships [9,22,40,45,46]. With reference to the identified relationships and orientation(s) of fracture cross-cutting and abutting, styles of cements and evident dissolution generations (Figures5–8), three stages of fracture development and cement precipitation in the fracture zones are suggested, and which may be consistent with three cycles of fault activity [29,30]. Over the long history of multi-stages tectonic and fault activities affecting the Ordovician Halahatang area carbonate sequence [29,30], four stages of diagenesis were documented in this study, in those carbonate rocks of the fault damage zones (see Figure9). Diagenetic settings can be subdivided into marine, eogenetic and mesogenetic environments [7,40,47]. The first-generation cements (C1-1) observed in the carbonate rocks are marked by fibrous cements (Figure6a), generally in the form of isopachous grain coatings, and are consistent with the marine environment in the Tarim Basin [28,40]. The subsequent granular and stalactitic cements (C1-2) (Figures4 and6a) are characterized by fabric-selective dissolution, suggesting an early eogenetic environment [ 40]. Although there are multiple stages of cementation in the intergranular porosity of the Ordovician carbonate rocks, most of the cementation occurred in a marine to eogenetic environment [28,40]. The intergranular cements are generally cross-cut by the fractures (Figure4b,c), as well as by the stylolites (Figure8b). Thus, most of the cements likely formed during the early stages of diagenesis, prior to faulting activity. In the shallow burial, eogenetic environment, compaction was initiated, and following two subsequent phases of bedding-parallel stylolite formation (S1-1 and S1-2), led to the sequence of carbonate rocks becoming progressively more compacted with depth. During the first episode of fracturing, tectonic stylolites commonly cut across horizontal stylolites (Figure5b,c) suggesting that fracture initiation began after S1-1 and S1-2 [ 41,42]. In the core and sample thin sections, the tectonic stylolites are cross-cut by later fractures. Some tectonic stylolites are reactivated by and absorbed into the later fractures (Figure6b). In general, the early fractures have Minerals 2018, 8, x FOR PEER REVIEW 9 of 15

Figure 8. Photographs showing dissolution in the carbonate fault damage zones in the Halahatang area (the pink-dye resin showing the porosity). (a) Multiple dissolution in cores, the first dissolution (D1) porosity is occluded by calcite cements, the second dissolution (D2) porosity and vugs are filled with cements and bitumen, the late fracture (under arrow of D2) reactivation and opening possibly suggest third stage dissolution; H601-1, core, 6638 m; (b) dissolution porosity along a stylolite, H601-18, thin section, 6755.04 m; (c) dissolution porosity among the cataclasites, H803, thin section, 6573.78 m.

5. Discussion

FigureFigure 9. Paragenetic 9. Paragenetic sequences sequences of diagenesis of diagenesis affecting aﬀecting the Ordovician the Ordovician carbonate carbonate rocks rocks in the in fault the fault damagedamage zones zones of the of Halahatang the Halahatang area. area. The The length length and and height height of ofeach each green green bar bar is isrepresentative representative of of the the relativerelative duration duration andand magnitudemagnitude of of event, event, respectively. respectively. The The diagenetic diagenetic sequences sequences from from stage stage 0 are 0 those are thosethat likely that tolikely have to occurred have occurred early, during early, burial during and burial compaction and compaction (i.e., an eodiagenetic (i.e., an eodiagenetic setting) prior to setting)the prior onset to of faultthe onset activity. of fault Some activity. stylolites Some (S1-2) st withylolites small (S1-2) dip with angles small to bedding dip angles possible to bedding indicate the possibleinitiation indicate of faultthe initiation activity. Stagesof fault I–III activity. possibly Stages correspond Ⅰ-Ⅲ possibly to the correspond fault activities to the occurring fault activities during the occurringLate Ordovician,during the Late Permian Ordovician, and Cenozoic Permian eras and (Figure Cenozoic2a) [29 eras,30 ].(Figure 2a) [29,30].

DiageneticIn the secondsettings stage can ofbe fracturing subdivided (Figure into 7ma), therine, later eogenetic fractures and (F2) mesogenetic generally were environments terminated or [7,40,47].distorted The first-generation at the intersection cements withearly (C1-1) fractures observed (F1). in the The carbonate fracture enlargementrocks are marked and dissolutionby fibrous are cementsubiquitous (Figure in 6a), the generally carbonate in rocks the form of the of studyisopachous area (Figure grain coatings,7b,c), and and subsequent are consistent cements with infilled the a marinenarrower environment aperture in than the forTarim the earlyBasin cements. [28,40]. TheseThe subsequent observations granular suggest and intense stalactitic fluid activity cements which (C1-2)had (Figures a dominant 4 and influence 6a) are on characterized the precipitation by offabric-selective calcite during thedissolution, carbonate suggesting diagenesis (cementation)an early eogeneticin the environment fault damage [40]. zones. Although This is consistentthere are mult withiple the stages documented of cementation extensive, in hydrothermalthe intergranular activity porosityduring of thethe PermianOrdovician throughout carbonate the rocks, Tarim most Basin of [48 the,49]. cementation Almost all ofoccurred the porosity in a inmarine the first to and eogeneticsecond environment fractures has [28,40]. been occludedThe intergranular by strong cements precipitation are generally of calcite cross-cut cements by during the fractures deep burial environment (Figures6 and7). We observed open/uncemented microfractures (F3) overprinting across all the earlier cemented fractures (Figure7), suggesting a latest stage fracture activity that is not related to strong diagenesis. The cross-cutting relationship for this episode possibly suggests that the tectonic stress field had changed, from the approximately NNE directional compression in the late Ordovician [29] to the NNW directional extension in the Eocene [25,27], for the new orientation fracture initiation. There is also observed fracture enlargement, but weak dissolution in this latest stage of fracture (Figures7b and8a). Following multiple phases of structural diagenesis (Figure9), late fractures can overprint those early fracture textures, modifying and/or inheriting from their pre-existing fracture traces and ultimately superimposing upon these diagenetic features a new phase of fracture activity.

5.2. Fault Controls on the Structural Diagenesis Together with the macro- and micro-structures observed in the Halahatang area Ordovician carbonates [28,40,48–50], there are multiple features that evidence the complicated structural control upon diagenesis rather than that which typiﬁes the relatively uniform diagenesis of the carbonate country rocks unaﬀected by faulting (Table1). The fracture elements present and production data [ 30] Minerals 2019, 9, 360 11 of 15 have shown the structure-related diagenesis is constrained to within the fault damage zones, or their immediate vicinity, with those rocks productivity controlled by the fault activities.

Table 1. A comparison of diagenesis between fault damage zones and country rocks.

Diagenesis Country Rocks Damage Zone Compaction relatively strong, line/serrate contacts multiple point/line/serrate contacts 2–3 stages, bedding-parallel and inclined 1–2 stages, serrate and wave-like profiles, Pressure solution stylolites and 1–2 stages tectonic stylolites, bedding-parallel stylolites; tight localized dissolution porosity and bitumen infill undeveloped, one set of regional 2–4 stages, multiple kinds and orientations of sub-vertical fracture, calcite Fracturing fractures, partial filling/cement, multiple-stages infill/cementation, little dissolution of of dissolution enhancing porosity porosity 1–2 stages of marine cements, 1–2 stages 2–5 stages eogenetic-mesogenetic cements, Cementation eogenetic-mesogenetic cements, strong relatively weak cementation and cementation and little dissolution multiple-stages of dissolution a few burial dissolution, dissolution 2–4 stages dissolution, development of partially Dissolution porosity occluded by cements filled porosity, vugs and caves

Previous studies have shown that the Ordovician limestones in the Tarim Basin have undergone more than 10 generations of diagenesis in the submarine, eogenetic and mesogenetic environments [28,40,48–50]. In these studies, multiple episodes of fracture activity and/or hydrothermal activity evidenced in the Ordovician carbonates are correlated to fault activities in the Tarim Basin [28,49,51]. Meteoric dissolution and cementation are sparse in the country rocks, although some are possible related to penecontemporaneous diagenesis [48,50] and most can be related to an unconformity [51]. In the Halahatang area, the arenopelitic fillings observed in the fault damage zone samples collected as part of this study suggest a fault related meteoric diagenesis. By comparison analysis, more than three additional stages of cementation and dissolution can be identified atop of those present from the burial diagenesis, with these likely being related to structural activity, as compared to most country rocks that generally had only 1-2 stages of cementation (Figures4a and6a), but little dissolution [ 28,40,48–51]. In this context, the complicated diagenetic processes are almost all related to fault activity, other than those resulting from the depositional and burial diagenesis in the Ordovician carbonate rocks. In the deep and long burial history, the host rocks generally had strong compaction, 1-2 stages of bedding-parallel stylolites, 1-2 stages of submarine and 1-2 stages of burial cementation, and little fracture and dissolution porosity in Halahatang area (Table1, Figures4–8). This is consistent with other areas in the Tarim Basin [28,40,48–51]. On the other hand, there are complicated compaction and pressure solution, multiple fracturing and cementation and dissolution along the fault damage zones of the Halahatang area (Table1). The limestones show a relatively uniform compaction with line contact in the grainstones and packstones in the country rocks [28,40], but both strong and weak compaction in the fault damage zones (Figure4b,c). The relatively strong compaction is not only related to lithology, but also possibly affected by the fault tilting and transpressional stress regime operating during the fault activity [29], whereas some rocks in the damage zones show relatively weak compaction with point/line contact between the grains (Figure4b). The weak compaction with grainstones floating in the cements is possibly related to slow subsidence and rapid cementation during the early fault activity related uplift. In addition, the inhomogeneous early cementation and overpressure fluid(s) may inhibit the compaction process [7,52]. Except for bedding-parallel stylolites, inclined, vertical and cross-cutting types of stylolites were observed in the fault damage zones (Figure5a–c), but were absent in the country rocks (Figure5d). In addition, the bedding-parallel stylolites in the damage zones have relatively larger amplitudes than those of the country rocks. These features suggest a stronger degree of pressure solution in fault damage zones. Tectonic (transverse) stylolites were mainly observed perpendicular to bedding, whereas, some inclined and irregular stylolites suggest complicated compressional stress in the fault damage zones. The stylolites are tight in the country rocks, but some stylolites were reactivated and Minerals 2019, 9, 360 12 of 15 have residual porosity (Figure5a,b). In addition, there are some evidence of dissolution porosity, and cements and bitumen infill along with the stylolites (Figure8b), suggesting a preferred pathway of the stylolites for fluid flow, corrosion and mineralization during this diagenetic stage. These reactivated stylolites in the damage zones are favorable for fluid interaction by locally increasing the porosity and permeability above that of the tight stylolites in country rocks. The reactivation of the stylolites is closely related to multiple faulting activities, while the origin and mechanism of these activities requires further study. Fracture processes have been systematically investigated along the fault zones [9,11,15,24]. In this area, there is generally sparse fractures (less than one in two meter cores) occurred from the boreholes in the country rocks. However, in the damage zones, there are multiple kinds (Figures5–7) and dense fractures with fracture frequency more than 50 in one meter cores [30]. The fractures mainly distributed in the fracture zone within 2 km of the fault zone [30,31]. Following multiple fault activities, the fractures are characterized by 2–3 periods reactivation, as well as multiple dissolution and cementation (Figures6–8). Three stages fracturing (Figure7) suggest three cycles of fracture opening and sealing. It is noteworthy that the latest fractures (F3) are almost opening, but the early fractures (F1 and F2) are mostly sealed. Nonetheless, the fractures varied in patterns, frequency, parasequence and petrophysical properties [30] with spatio-temporal change in the fault zones. These suggest strong anisotropy and heterogeneity of the fracturing and associated diagenesis in the fault zones. In the fault damage zones, there are complicated carbonate cementation and dissolution features observed (Figures6–8). Except for the early depositional diagenesis, the later three stages of cementation and dissolution may be related to the fault activity in the fault damage zones (Figure9). Previous studies have documented 2–3 generations of strong cementation among the grainstones, but little dissolution in the country rocks [28,40]. In the fault damage zones, additional 3 stages of calcite cementation have pervaded the fractures and cataclastic rocks (Figures6 and7). The strong cementation of early two stages cements (C2 and C3) may correspond to a rapid subsidence following early fault activities [28–30]. In addition, the hydrothermal activity possible increased the calcite precipitation [48,49,51], considering the damage zone is favorable for fluid flow and mineral precipitation [5,14,17]. On the other hand, the latest fracture family (F3) is possible related to Eocene fault activity, prior to the onset of large scale cementation. In the Halahatang area, most primary porosity has been occluded by the strong cementation during the long burial history of these rocks (Figure4)[ 28,40]. The microstructures present show more than 90% porosity is of dissolution porosity (Figures5–8). This is consistent with the fact that the Ordovician carbonate reservoirs are mainly characterized by dissolution vugs and caves [28,33,34], and almost all occur along the fault damage zones [30,31]. Regardless of the intense cementation, there is much stronger dissolution in the fault damage zones (Figure8) than the country rocks (Figures4a and6a). This suggests the dissolution porosity mainly occurred within the fault damage zones, although it is still ambiguous for the process of the large caves [34]. There is generally a complicated interplay between cementation and dissolution as a consequence of mechanical deformation in the Ordovician carbonates [7,13,14,53]. In this study, much more dissolution developed along the fault damage zones, suggesting fluid pathways formed in those areas facilitating the later dissolution in the fault zones.

6. Conclusions Although the carbonate diagenesis within fault damage zones is diﬃcult to predict in the subsurface, we propose the following model, based upon an integrated analysis of the carbonate fault damage zones in the Tarim Basin (Halahatang area).

1. Multiple phases of carbonate diagenesis in the fault damage zones may be identiﬁed from the subsurface based upon stylolite type and fracture cross-cutting relationships, fracture orientations and cementation generations in the sedimentary basin strata. 2. The country rocks have undergone strong compaction, pressure solution and cementation, during deep burial, but few fracture and dissolution porosity features are evident in the Ordovician Minerals 2019, 9, 360 13 of 15

carbonates in this study area. However, there are complicated compaction, 1-2 stages of tectonic stylolites, 2-4 stages fractures and associated cementation and dissolution in these same carbonates found along the fault damage zones. 3. In contrast to the country rocks, this study suggests an additional three cycles of structural diagenesis occurred within the carbonate fault damage zones. The two early stages of fracture activities are followed by strong cementation, while much more open fracture is characteristic of the latest stage of brittle deformation. 4. The carbonate structure related diagenesis is constrained to the fault zones, and its spatio-temporal distribution and variation are mainly controlled by the three diﬀerent stages of fault activities aﬀecting these rocks.

Author Contributions: G.W., Y.Z. and E.X. conceived the study; E.X., X.L. and C.S. collected samples and analyzed the data; G.W., Y.Z. and H.Q. discussed and interpreted the results; G.W. and H.Q. wrote the paper with contribution from all authors. Funding: National Natural Science Foundation of China (Grant No. 41472103) and National Science and Technology Major Project (2011ZX05004001). Acknowledgments: We thank the Tarim Oil Company for data support and project research. We are grateful to Professor Ian Coulson and DCP Peacock for their constructive comments and careful revisions of the manuscript that substantially improved the manuscript. We also thank Lixin Chen, Xiaoguo Wan, Guohui Li, Kuanzhi Zhao and Lianhua Gao for their help for data analysis and interpretation. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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