Oil Generation Induces Sparry Calcite Formation in Lacustrine Mudrock, Eocene of East China

Marine and Petroleum Geology 71 (2016) 344e359

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Marine and Petroleum Geology

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Research paper Oil generation induces sparry calcite formation in lacustrine mudrock, Eocene of east China

* Jianguo Zhang a, b, Zaixing Jiang a, b, , Xiaolong Jiang c, Siqi Wang a, Chao Liang d, Minghao Wu a a College of Energy, China University of Geosciences, Beijing 100083, China b Institute of Earth Science, China University of Geosciences, Beijing, 100083, China c School of Earth and Mineral Sciences, Pennsylvanian State University, PA 16803, USA d School of Geosciences, China University of Petroleum, Qingdao 266580, China article info abstract

Article history: Sparry calcite is common in sedimentary rocks. In mudrocks, it has been reported to occur as veins in Received 28 September 2015 bedding-parallel, oblique, and vertical fractures. Tectonic forces were traditionally suggested to be Received in revised form responsible for the calcite vein formation. Some recent studies showed that hydrocarbon generation 3 December 2015 could be responsible for forming bedding-parallel calcite veins of fibrous sparry calcite; however, the Accepted 10 January 2016 process of sparry calcite formation by hydrocarbon generation is not well understood. Available online 13 January 2016 Sparry calcite is common in the Eocene mudrocks of the Zhanhua and Dongying Sags of the Bohai Bay Basin and the Biyang Sag of the Nanxiang Basin, east China. In this study, the petrology, carbon and Keywords: fl Oil generation oxygen isotopes, minor geochemistry, uid inclusion, and drill stem testing characteristics of sparry Sparry calcite calcite and/or sparry calcite-bearing intervals in mudrocks were investigated. Oil generation could lead to Mudrock overpressure, inducing accommodation space for the precipitation of sparry calcite, while organic acids, a Eocene by-product of oil generation, promoted the dissolution of micrite, providing an ion source for diagenetic East China sparry calcite growth. Structureless mudstone can experience elastic deformation before fracture generation, forming equant and dispersed sparry calcite. Laminated shale, on the other hand, easily reaches the yield point because of its anisotropic nature. An upward seepage force by fluid overpressure can induce bedding-parallel fractures, resulting in three stages of sparry calcite formation characterized by: 1) discrete and equant sparry calcite, 2) horizontally intergrown and equant sparry calcite, and 3) horizontally intergrown and fibrous sparry calcite. Thus oil generation can be responsible for sparry calcite formation, establishing some new characteristics for this process in mudrocks, including: 1) sparry calcite could also form during the process of elastic deformation as well as fracturing in mudrock and 2) fibrous sparry calcite began its growth from equant sparry calcite. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction compression or extension enables fractures for forming space for veins (e.g., Worden et al., 2015). In recent studies, the relationship Sparry calcite is defined as diagenetic calcite crystals with a size between calcite vein formation and hydrocarbon generation larger than 50 mm(Scholle and Ulmer-Scholle, 2003), widely received wide attention. Bedding-parallel veins of fibrous sparry occurring in sedimentary rocks with different morphologies calcite are abundant in and around petroleum source rocks in the (Tucker and Wright, 1991). In mudrocks, it was reported to occur as Neuquen Basin, Argentina (Parnell and Carey, 1995; Parnell et al., veins in bedding-parallel, oblique, and vertical fractures (e.g., Bons 2000; Zanella et al., 2015a), the Wessex Basin, southern England et al., 2012). Tectonic forces were traditionally suggested to be (Zanella et al., 2015b), the foothills of Magallanes-Austral Basin, responsible for calcite vein formation, in which structural southern Chile and Argentina (Zanella et al., 2014a), and the Paris Basin, France (Cobbold et al., 2015). Oil inclusions occur in bedding- parallel veins in the Prague Basin, Czech Republic (Dobes et al., * Corresponding author. College of Energy, China University of Geosciences, 1999; Suchy et al., 2002), the Posidonia Shale of NW Germany Beijing 100083, China. (Jochum et al., 1995), and the Neuquen Basin, Argentina (Rodrigues E-mail address: [email protected] (Z. Jiang). http://dx.doi.org/10.1016/j.marpetgeo.2016.01.007 0264-8172/© 2016 Elsevier Ltd. All rights reserved. J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359 345 et al., 2009). Some studies have suggested that hydrocarbon gen- The two areas have experienced a burial history typical for eration is an important triggering mechanism of bedding-parallel extensional to transtensional basins, with dominant subsidence veins of sparry calcite formation (e.g., Cobbold et al., 2013). How- and occasional uplift (Fig. 3A, B) (Qiu et al., 2006). The burial rate is ever, a systematic study of the characteristics and formation rapid during Paleogene period, followed by relatively gradual burial mechanism of sparry calcite by hydrocarbon generation (e.g., trig- during the Neogene period in the two basins (Fig. 3A, B) (Li et al., gering mechanism, ion source, accommodation space, and crystal 2003). Currently, the mudrocks are thermally mature in the up- maturation) is needed in mudrocks. per Es4 and lower Es3 members of the Bohai Bay Basin (Fig. 3A) Sparry calcite is common in the Eocene mudrocks from the (Zhang et al., 2009) and in the lower Eh3 member of the Nanxiang Zhanhua and Dongying Sags of the Bohai Bay Basin and the Biyang Basin (Fig. 3B) (Jiang et al., 2013). Sag of the Nanxiang Basin, east China. The sparry calcite shows some diagnostic characteristics, which have been rarely reported in 3. Data and methods previous studies (e.g., Bons et al., 2012; Hilgers et al., 2001), including: 1) dispersed and random equant sparry calcite common The cores for this study were collected from wells F1, L1, L67, L69 in the mudrock, 2) bedding-parallel-arranged sparry calcite later- and N1 in the Es3 and Es4 members of the Shahejie Formation of the ally dispersed besides being laterally intergrown, and 3) mudrock Zhanhua and Dongying Sags of the Bohai Bay Basin (Figs. 1A, 2; texture related to organization and distribution of sparry calcite. Table 1), and wells B1 and C2 in the Eh3 member of the Heotaoyuan The above features make these Eocene mudrocks an appropriate Formation of the Biyang Sag of the Nanxiang Basin (Figs. 1B, 2). study subject to enrich our understanding of sparry calcite in fine- Geologic observations and sampling cover 1216 m of cores with grained rocks. drill stem testing (DST) analysis from five wells, total organic car- In this study, we describe characteristics of the sparry calcite bon (TOC) analysis from 625 samples, Rock-Eval analysis from 367 and/or sparry calcite-bearing intervals in these Chinese Eocene samples, vitrinite reflectance (Ro %) analysis from 56 samples, Mg, mudrocks (i.e., petrology, geochemistry, fluid inclusions, and Ca, Mn, and Sr ratios in elemental geochemistry from 80 samples, pressure conditions) in detail, and analyze the origin of the sparry stable oxygen and carbon isotope analysis of micrite and sparry calcite, including time of formation, source ions for precipitation, calcite from 22 samples, high-resolution field emission scanning space for sparry calcite growth, and crystal maturation over time electron microscopy (FESEM) observation from 23 samples, chlo- for the sparry calcite. Oil generation is suggested to be responsible roform asphalt “A” analysis from 25 samples, and co-existing for the sparry calcite formation in the Bohai Bay and Nanxiang aqueous and petroleum inclusion analysis from four thin sections Basins of east China, showing the relationship between sparry (Figs. 3e13; Table 2). Sampling for DST, TOC, Rock-Eval, and vitrinite calcite formation and hydrocarbon generation. reflectance on the Zhanhua and Dongying Sags and Biyang Sag were collected at the Shengli Oil field and Henan Oil field. All an- 2. Geologic background alyses were performed in the Geological Process and Mineral Re- sources state key laboratory at China University of Geosciences The Bohai Bay and Nanxiang Basins are oil-bearing, extensional (Beijing). to transtensional Cenozoic basins in east China (see Allen et al., The core images were obtained from a high-resolution scanning 1998). This research focuses on the Eocene Zhanhua and Dongy- apparatus, working at a voltage of 260 V. The cores were cut in half ing Sags of the Bohai Bay Basin, and the Biyang Sag of the Nanxiang vertically along the length of the cylinder before the scanning. The Basin (Fig. 1A, B). The Zhanhua Sag and Dongying Sag cover an area thin sections for fluid inclusion were polished on both sides, which of 2800 and 5700 square kilometers, respectively. The Biyang Sag is were observed in a polarized microscope to define the fluid inclu- approximately 1000 square kilometers in area. Thick lacustrine sion assemblages. The fluid inclusion microthermometry was car- mudrocks developed in the fourth (Es4) and third (Es3) members of ried out in a heating-cooling stage to determine the the Shahejie Formation of the Zhanhua and Dongying Sags (Fig. 2A) homogenization temperature of coeval aqueous and petroleum (Zhang et al., 2009), and in the third (Eh3) member of the inclusions. Samples for oxygen and carbon isotopic analyses were Hetaoyuan Formation of the Biyang Sag (Fig. 2B) (Jiang et al., 2014). loaded in sealed reaction vessels with sample size between 0.1 and

Fig. 1. (A) Geologic setting of the Zhanhua and Dongying Sags in the Eocene of the Bohai Bay Basin, east China. (B) Geologic setting of the Biyang Sag in the Eocene of the Nanxiang Basin, east China. The locations of cored wells are designated by small crosses. 346 J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359

Fig. 2. Tertiary stratigraphy and paleoenvironment interpretation of the Zhanghua and Dongying Sags in the Bohai Bay Basin, and the Biyang Sag in the Nanxiang Basin. The strata used in this study are designated by a black line in the lithology column.

0.3 mg, flushed with helium gas, and reacted at 72 C. The evolved 4.1. Petrology characteristics carbon dioxide in the vial headspace was sampled using a Finnigan Gas-Bench, and isotope ratios were measured by a Finnigan MAT Cores in this study contain laminated shale (Fig. 4A) and Delta þ XL mass spectrometer. Replicate analyses of NBS-19 and structureless mudstone (Fig. 5A). Calcite morphologies in these MERCK carbonate standards yielded a precision for carbon and mudrocks are primarily micrite (Fig. 6) (calcite crystals smaller than oxygen isotopes at ±0.2‰ or better. All isotopic results were re- 4 mm; Folk, 1965) with secondary sparry calcite. ported in standard delta notation and corrected to PDB. The isotope The laminated shale contains alternating light-colored micrite ratio values of micrite as well were analyzed on carbonate samples laminae (dozens to hundreds of micrometers in thickness) and without sparry calcite. Some pretreatments were conducted to dark-colored clay and organic matter laminae (dozens to hundreds collect sparry calcite for elemental and isotope geochemistry ana- of micrometers in thickness) in thin section (Fig. 4B). The sparry lyses. The bedding-parallel sparry calcite was collected from the calcite is categorized into three types: 1) discrete and equant sparry bedding-parallel calcite veins. To collect the dispersed and random calcite crystals with a long axis of 50e100 mm in length (Fig. 4C), 2) sparry calcite, samples were soaked in 24% HF for 48 h to remove horizontally intergrown and equant sparry calcite with a long axis the siliceous particles, and then a 100-mesh griddle was used to of 100e200 mm in length (Fig. 4D), and 3) horizontally intergrown remove the micrite. In addition, the samples for isotope analysis and fibrous sparry calcite with a long axis of 200e1000 mmin were pretreated with 1% sodium solution to remove the organic length (Fig. 4E). The size and number of the sparry calcite crystals matter, which interferes with the accuracy of isotope ratios. gradually increase from type one through three (Fig. 4CeE). In addition, the sparry calcite locally occurs within micritic laminae 4. Results (Fig. 4C). The fibrous forms show characteristics of growth competition, with the width of some crystals increasing upward All data collected are summarized in Table 3. Each analysis is while truncating neighboring crystals (Fig. 4E). presented separately below. In structureless mudstone, the bedding-parallel veins of sparry J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359 347

Fig. 3. (A) Thermal and burial history plot for well L69. See Fig. 1A for well location. (B) Thermal and burial history plot for well B1. See Fig. 1B for well location. Early rifting subsidence is followed by slow burial during Neogene and Quaternary times in the above areas.

Table 1 Core data from the Bohai Bay Basin (Zhanhua and Dongying Sags) and the Biyang Sag of the Nanxiang Basin of east China (Eocene) for this study. See Fig. 1 for well locations.

Well Location Strata Core thickness (m) Thickness with sparry calcite (m)

L67 Zhanhua Sag Eocene 42 7 L69 Zhanhua Sag Eocene 231 17 N1 Dongying Sag Eocene 208 19 F1 Dongying Sag Eocene 414 13 L1 Dongying Sag Eocene 218 14 B1 Biyang Sag Eocene 36 8 C2 Biyang Sag Eocene 72 5

Table 2 sparry calcite crystals with an equant shape (Fig. 5CeD). Length of Drill stem testing (DST) and pressure coefficients in mudrock intervals with sparry the equant and dispersed sparry calcite crystals is 50e300 mm calcite in wells from the Zhanhua and Dongying Sags of the Bohai Bay Basin and the (Fig. 5CeE). Contrary to bedding-parallel-arranged sparry calcite, Biyang Sag of the Nanxiang Basin, east China (Eocene). See Fig. 1 for well locations. the shapes of dispersed and random crystals do not show any size Well L67 L69 F1 N1 L1 differentiation (Fig. 5CeE). In thin section, the bedding-parallel- Depth (m) 3299.7 3053 3205 3430 3635 arranged sparry calcite commonly co-occurs with equant and Strata pressure (MP) 49.93 54.45 45.75 55.02 54.17 dispersed ones (Fig. 5D). In addition, sparry calcite can locally form pressure coefficient 1.54 1.82 1.43 1.60 1.49 along the rim of organic matter (Fig. 5E). calcite can also be observed (Fig. 5A), but their horizontal conti- 4.2. Geochemistry characteristics nuity is not as good as in the laminated shale. The minerals are mostly chaotic and disorganized in structureless mudstone Cores in this study were always slightly higher than 0.6% in (Fig. 5B). Correspondingly, there are also some randomly dispersed vitrinite reflectance (Figs. 7e9), which suggests a thermally mature oil window for these Eocene rocks. A vitrinite reflectance of 348 J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359

Table 3 Summary data with corresponding location in article supporting the hypothesis that oil generation induces sparry calcite formation. HT ¼ homogenization temperature; D PC ¼ pressure coefficient; DST ¼ drill stem testing; VR ¼ vitrinite reflectance; TOC ¼ total organic carbon. Though the vitrinite reflectance also reaches a thermally mature oil window in non-sparry calcite-bearing intervals, the low TOC content reduces the ability to induce sparry calcite formation (see Fig. 12 for explanation). See Fig. 1 for location of wells in the Eocene Bohai Bay and Nanxiang Basins of east China.

Data Sparry calcite Micrite Section Figure/Table

Thin-section Sparry calcite bodies are locally embedded in the micritic laminae. 4.1 Fig. 4C Sparry calcite bodies are distributed around the rims of organic matter. Fig. 5A Well L69 B1 L69 B1 Mg/Ca (10 2) 1.5e3.1 (avg.2.2) 5.3e12.4 (avg.8.6) 3.2e10.4 (avg.7.5) 18.1e80.7 (avg.36.2) 4.2 Figs. 7e8 Mn/Ca (10 3) 0.1e0.7 (avg.0.3) 2.1e16.2 (avg.6.3) 0.9e3.4 (avg.2.6) 22.6e64.3 (avg.40.7) Sr/Ca (10 2) 1.1e2.4 (avg.1.4) 5.4e23.8 (avg.10.7) 0.4e0.6 (avg.0.5) 0.30e0.62 (avg.0.44) d13C(‰) 2.9 to 4.6 (avg. 3.9) 1.5 to 3.8 (avg. 2.8) 2.4 to 4.4 (avg. 3.5) 1.4 to 4.3 (avg. 3.1) d18O(‰) 12.6 to 9.8 12.3 to 8.7 10.5 to 8.4 (avg. 9.6) 9.4 to 7.1 (avg. 8.2) (avg.11.6) (avg. 10.3) HT (C) 87.5e128.6 (avg. 83.2e113.4 (avg. 4.3 107.9) 102.2) Sparry calcite-bearing interval Non-sparry calcite-bearing interval

Well L67 L69 F1 N1 L1 B1 PC Fluid inclusion 1.39 1.36 4.3 Fig. 10 DST 1.54 1.82 1.43 1.60 1.49 4.4 Table 2 TOC (wt.%) >2.0 <2.0 4.2 Figs. 7e9 D VR (Ro %) >0.6 >0.6 4.2 Figs. 7e9

Fig. 4. Core and thin section characteristics of sparry calcite in a laminated shale from well F1 in the Dongying Sag of Bohai Basin, east China (see Fig. 1 for location of well F1). (A) 3165.60e3165.90 m. Laminated shale with horizontal calcite veins (red arrow). The sample locations of BeE are shown in this core image. See Fig. 13A for a close-up image within the blue rectangle. (B) 3165.80 m. The light-colored laminae (next to the gray bar at the right side of the figure) are composed of micrite and dark-colored laminae are composed of organic matter and clay (next to the black bar at the right side of the figure). (C) 3165.72 m. The micrite laminae contain equant sparry calcite crystals (red arrow), which are mostly discrete and 50e100 mm long. (D) 3165.60 m. Laminae composed of intergrown and equant sparry calcite crystals 100e200 mm in length. (E) 3165.65 m. The bedding-parallel sparry calcite is laterally intergrown and shows fibrous morphology along its long axis vertical. Size of the sparry calcite crystals reaches 1000 mm in length. Some crystals gradually become larger upwards (red arrows), but some crystals gradually pinch out (blue arrows). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

0.6e1.2% is present in a thermally mature oil window (Peters et al., 2.9e4.6‰ (avg. 3.9) and 1.5e3.8‰ (avg. 2.8), and the micrite iso- 2005). Sparry calcite only occurs in samples with high TOC contents topic ratios were 2.4e4.4‰ (avg. 3.5) and 1.4e4.3‰ (avg. 3.1) in (higher than 2 wt.%), high S1þS2 values, and high chloroform samples from the Zhanhua and Dongying Sags of the Bohai Bay asphalt “A” values (Figs. 7e9), suggesting high potential for oil Basin, and the Biyang Sag of the Nanxiang Basin, respectively generation. In the elemental geochemistry data, sparry calcite al- (Figs. 7e8). The sparry calcite d18O isotopic ratios were 12.6 ways has lower ratios of Sr/Ca, but higher ratios of Mg/Ca and Mn/ to 9.8‰ (avg. 11.6) and 12.3 to 8.7‰ (avg. 10.3), and the Ca, than micrite (Fig. 7). The sparry calcited13C values were micrite ratios were 10.5 to 8.4‰ (avg. 9.6) and 9.4 to 7.1‰ J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359 349

Fig. 5. Core and thin section characteristics of sparry calcite in the structureless mudstone of wells L1 and L69 in the Dongying and Zhanhua Sags, respectively. See Fig. 1 for location of the wells. (A) 3631.00e3631.30 m, well L1. Bedding-parallel calcite veins are present in the mudstone. The sample locations of Fig. 5BeD are shown in this core image. See Fig. 13BeC for close-up images within the blue and red rectangles. Note that the vertical fractures were formed after core collection because of the sharp decrease in overburden gravity force. (B) 3631.25 m, well L1. The minerals are disorganized and chaotic. (C) 3631.12 m, well L1. Dispersed and relatively random sparry calcite crystals with equant shapes. (D) 3631.18 m, well L1. Note the co-occurrence of dispersed and random equant crystals and the calcite lenses composed of laterally intergrown fibrous shapes in the same visual field. (E) 3057.52 m, well L69. Sparry calcite bodies distributed around the rims of organic matter (red arrow). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. High-resolution ﬁeld emission scanning electron microscopy (FESEM) image of micrite (left) and its composition (right) from 3057.55 m, well L69 (see Fig. 1 for location in the Zhanhua Sag).

(avg. 8.2) in samples from the Zhanhua and Dongying Sags of the paleopressures at the time of calcite precipitation. Confocal Laser Bohai Bay Basin, and the Biyang Sag of the Nanxiang Basin, Scanning Microscopy (CLSM) was used to determine volumetric respectively (Figs. 7e8). The sparry calcite and micrite samples liquidevapor ratios of petroleum trapped within each of the fluid showed little difference in d13C values, whereas the sparry calcite inclusions. Using the PressureeVolumeeTemperature (PVT) isotopic oxygen is more depleted than that in micrite (Figs. 7e8). modeling software Vtflinc (Aplin et al., 1999), these liquidevapor ratios were used to calculate the pressure conditions of the petro- 4.3. Fluid inclusion characteristics leum in each inclusion at its homogenization temperature (Fig. 10A, B). These conditions represent the minimum temperature and The hydrocarbon and aqueous fluid inclusions trapped in the pressure at which the petroleum was trapped. The physical prop- sparry calcite grains, combined with reconstruction of the tem- erties of the petroleum were used to construct the hydrocarbon perature history, can be used to assess the paleotemperatures and isochore, along which the true trapping temperature and pressure 350 J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359

Fig. 7. Lithology and geochemistry characteristics of well B1 in the Biyang Sag of the Nanxiang Basin, east China (see Fig. 1 for well location). The core intervals with sparry calcite are labeled with a black line in the lithology column. occur at the intersection with the aqueous inclusion data (Fig. 10C, 5. Discussion D). The formation waters in equilibrium with petroleum in a multi- phase system were methane saturated, and hence the homogeni- 5.1. Period of formation zation temperature was assumed to be the trapping temperature (e.g., Swarbrick, 1994). The true trapping pressures and tempera- Sparry calcite is a diagenetic calcite crystal (Scholle and Ulmer- tures were therefore estimated where possible using the homog- Scholle, 2003; Gierlowski-Kordesch, 2010, p. 25). In this study, the enization temperatures of co-existing aqueous inclusions in sparry formation temperature of the sparry calcite crystals can be calcite crystals at 83.2e128.6 C (peak value at 102.2e107.9 C) approximately estimated by the co-existing aqueous inclusions, (Fig. 10A, B). The pressure was estimated to be 33.2 MP at 2436.5 m which was determined to be 83.2e128.6 C (peak value at (well B1) and 42.4 MP at 3048.5 m (well L69) (Fig. 10C, D), in which 102.2e107.9 C). the pressure coefficient is 1.36 and 1.39, respectively. It is regarded The geothermal gradient is suggested to be 35e45 C/km in the as an abnormally high pressure if the pressure coefficient is higher Eocene Es3 strata of the Zhanhua and Dongying Sags, and 38e51 C/ than 1.2 (Slavin and Smimova, 1998). Hence, the fluid inclusion data km in the Eocene Eh3 strata of Biyang Sag (Qiu et al., 2004, 2006). suggest overpressured environments. According to the typical thermal and burial history plot of the two areas (Fig. 3A, B), the sparry calcite crystals should approximately form if the depth is deeper than 2.0 km. Other work in the study 4.4. Drill stem testing (DST) area also lends support to this interpretation. For example, Wang et al. (2005) investigated the mudrocks at different strata in the DST is among the most reliable tests to speculate on strata Dongying Sag of the Bohai Bay Basin and discovered that sparry pressure (e.g., Swarbrick et al., 2000). DST data of the Eocene suc- calcite crystals did not occur where the burial depth was shallower cessions studied in east China yielded strata pressure ranging from than 3000 m. In addition, the vitrinite reflectance typically fell to 45.75 to 55.02 MP, and the pressure coefficient ranging from 1.43 to 0.5e0.7% in the depth intervals of 2.0e3.5 km, with a thermally 1.82 in intervals with sparry calcite (Table 2). The pressure coeffi- mature oil window suggested to be at a vitrinite reflectance of cient is much higher than the threshold of overpressure (1.2: Slavin 0.5e1.2% (after Peters et al., 2005). Hence, the formation of sparry and Smimova, 1998), thus the DST data also suggest that the in- calcite crystals matches the thermally mature oil window from tervals with sparry calcite are overpressured. J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359 351

Fig. 8. (A) Lithology and geochemistry characteristics of well L69 in the Zhanhua Sag of the Bohai Bay Basin, east China (see Fig. 1 for location). (B) Elemental geochemistry data and stable isotope values of sparry calcite and micrite of the interval between 3040 m and 3067 m. The interval with sparry calcite is labeled with a black bar in the lithology column. other workers as well. to the organic matter present. Kerogen maturation is able to generate large volumes of CO2 and organic acids (Barth and Bjørlykke, 1993). Two recrystallization 5.2. Triggering mechanism mechanisms are possible, one driven by CO2 and the other by acetic acid (see Meshri, 1986). Hydrolysis of CO2 produces carbonic acid, Both organic and inorganic mechanisms (i.e., meteoric water, which then dissociates to generate protons that are consumed in connate water, brines) have been reported to be potentially the dissolution-re-precipitation of calcite via equations (1)e(3) responsible for sparry calcite formation (e.g., Moore, 2001). In this below (from Irwin et al., 1977). Recrystallized calcite driven by study, the sparry calcite occurrences exactly match the Eocene core carbonic acids would incorporate certainly at least half of the car- e intervals with high TOC content (Figs. 7 9) and are located along bon from organic matter (equations (1)e(3)). The alternative the rims of organic matter (Fig. 5E). In addition, the occurrence of mechanism involves the dissociation of organic acids (primarily hydrocarbon inclusions in the sparry calcite crystals also suggests a acetic acid) to yield protons that drive the dissolution and re- direct physical relationship between sparry calcite crystals and precipitation of calcite via equations (4) and (5) (from Heydari organic matter. On the other hand, the scenarios of meteoric water, and Wade, 2002). Note that recrystallized calcite by organic acids connate water, and brine as the source for calcium ions are not able receives all of its carbon from the dissolving carbonate, conserving to explain fully why all the sparry calcite-bearing intervals exactly the original d13C compositions (equations (4) and (5)). The pro- match high TOC content. It should also be noted that diagenetic portion of carbon from carbonic acids in the sparry calcite can be calcite forming in meteoric environments without the presence of determined by equation (6) (from Worden et al., 2015). This organic organic matter commonly are depleted in 13C (e.g., Armstrong- 13 matter-derived CO2 typically has a d Cof25‰ relative to PDB 13 Altrin et al., 2008), whereas the d C values of the sparry calcite (Emery and Robinson, 1993). The sparry calcited13C values are 3.9 ‰ in the Eocene mudrocks of east China are high (1.4 and 4.4 and 2.8‰ in samples from the Zhanhua and Dongying Sags of the relative to PDB) (Figs. 7e8). In addition, the depth intervals of Bohai Bay Basin, and the Biyang Sag of the Nanxiang Basin, sparry calcite are within the thermally mature oil window. For respectively (Figs. 7e8). The micrite d13C values are 3.5 and 3.1‰, these reasons, we suggest formation of the sparry calcite is related 352 J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359

þ þ ⇔ 2þ þ H CaCO3 Ca CO3 (3) Dissolution and re-precipitation by organic acids

þ Organic acids⇔H þ CH3COO ; (4)

þ þ ⇔ 2þ þ H CaCO3 Ca HCO3 (5) Proportion of carbon from carbonic acids in the sparry calcite

d13Cðsparry calciteÞd13CðmicriteÞ ¼ (6) 13 13 d Cðsource rock derived CO2Þd CðmicriteÞ

5.3. Ion source for sparry calcite formation

The permeability in mudrock is generally less than 1000 nano- darcy without the occurrence of fractures (Jarvie, 2012), making it difficult for fluid movement. Fracturing is rare in cores and thin sections of the Chinese Eocene mudrocks, suggesting low permeability. Hence, external fluids would be very hard to inject into these mudrocks, showing that the ions for sparry calcite growth should be sourced from the mudrock itself. The mudrock is mainly composed of micrite, clay, and quartz, in which micrite accounts for half of the minerals (Jiang et al., 2013). Organic acids are able to dissolve micrite (Barth and Bjørlykke, 1993), providing ions for sparry calcite formation. Petrology and geochemistry data can certify that micrite dissolution provided ions for sparry calcite formation. Firstly, sparry calcite bodies are locally embedded in the micritic laminae (Fig. 4C), indicating sparry calcite forms from micrite. Secondly, the difference in Mg/Ca, Mn/Ca, and Sr/Ca ratios between sparry calcite and micrite samples also lends support to the interpretation (Figs. 7e8). Since hydrocarbon is expelled from the source rock during oil generation (Peters et al., 2005), some of the ions dis- solved from the micritic calcite were probably discharged along with the hydrocarbons. The solubility of MgCO3 and MnCO3 is higher than that of CaCO3, whereas the solubility of SrCO3 is lower 2þ than that of CaCO3 (Morse and Mackenzie, 1991). Hence, Mg and þ þ Mn2 ions are easier, whereas Sr2 ions are harder as compared to þ Ca2 ions, to dissolve in solution and mobilize along with the hydrocarbon expulsion. Thus, the remaining solution in the mudrock should have decreased Mg/Ca and Mn/Ca ratios and an increased Fig. 9. Lithology and geochemistry characteristics of well N1 in the Dongying Sag of the Bohai Bay Basin, east China (see Fig. 1 for location). The mudrock intervals with Sr/Ca ratio. Since the sparry calcite was recrystallized from the sparry calcite are labeled with black bars in the lithology column. remaining solution, it should also have lower Mg/Ca and Mn/Ca ratios and a higher Sr/Ca ratio (Figs. 7e8). Thirdly, d18O data can help to elucidate the precipitation sequence between micrite and respectively (Figs. 7e8). With these data inserted into equation (6), sparry calcite within these Eocene fine-grained rocks. Since the it is concluded that no more than 2% of the carbon in the sparry injection of external fluids is not an option, then the d18O value of calcite came from carbonic acids. This implies organic acids rather the mudrocks should fractionate in relation to the formation tem- than carbonic acids were the dominant triggering mechanism for perature, as represented in the oxygen isotopic ratios recorded in sparry calcite formation. In a previous study, Heydari and Wade the resultant sparry calcite (after Irwin et al., 1977; Sass et al., 1991). (2002) also observed that the recrystallization of calcite in a ther- Because sparry calcite is depleted in d18O values in comparison to mally mature oil window was induced by organic acids rather than micrite, the d18O data show sparry calcite formed at higher tem- carbonic acids. peratures. The burial history in the study areas is dominantly Dissolution and re-precipitation by carbonic acids subsidence (Qiu et al., 2004, 2006)(Fig. 3A, B), thus the strata temperature gradually rose along with burial. Sparry calcite formed CO2 þ H2O⇔H2CO3; (1) later than micrite, and so micrite must have acted as the ion source.

⇔ þ þ 2; 5.4. Formation of accommodation space H2CO3 2H CO3 (2) The small pore size of mudrock (typically several nanometers to J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359 353

Fig. 10. (A) Homogenization temperature of coeval petroleum and aqueous inclusions from 2436.5 m, well B1 in the Eocene of the Nanxiang Basin, east China. For location of well, see Fig. 1 (B) Homogenization temperature of coeval petroleum and aqueous inclusions from 3048.5 m, well L69 in the Eocene of the Bohai Bay Basin, east China. For location of well, see Fig. 1 (C) Calculation of the true trapping temperature and pressure of two coexisting fluid inclusions, using the intersecting isochores technique. The sample was collected in the same place as that of Fig. 10A. (D) Calculation of the true trapping temperature and pressure of two coexisting fluid inclusions, using the intersecting isochores technique. The sample was collected in the same place as that of Fig. 10B. several micrometers; classification by Loucks et al., 2012) is not formation. The integrated burial history, thermal history, and ho- sufficient to accommodate the growth of sparry calcite. Hence, mogenization temperatures of the fluid inclusions were shown in certain mechanisms must have induced the storage space for sparry Fig. 3A, B, in which the intersection between thermal history and calcite formation. Overpressure is essential for enlarging the ac- homogenization temperature (red (in the web version) dot in commodation space in mudrock, which is subject to horizontal Fig. 3A, B) represents the time and depth of sparry calcite formation compression and fluid overpressure (e.g., Rodrigues et al., 2009). (Wang, 2012). It shows that the sparry calcite crystals formed Fluid overpressure does exist in the sparry calcite-bearing intervals during the period of subsidence rather than uplift in the Bohai Bay as indicated by the fluid inclusion (Fig. 10) and DST data (Table 2). Basin and Nanxiang Basin. Previous studies also confirm that overpressure widely occurred in Fracture-opening mechanism also influences the properties of the studied strata in the three sags (Hao et al., 2007; Luo, 2014). fibrous sparry calcite. Rodrigues et al. (2009) distinguished two Disequilibrium compaction, tectonic compression, and hydrocar- phases of sparry calcite in bedding-parallel calcite veins in the Vaca bon generation are the three main overpressure mechanisms in Muerta Formation, Neuquen Basin: phase 1 was formed by oil mudrock. generation and phase 2 by horizontal compression. The long axes of Disequilibrium compaction was previously proposed as an the crystals in phase 1 are vertically oriented, similar to those in this important overpressure mechanism in the three sags (Xie et al., study (Figs. 4E, 5D). However, the calcite crystals in phase 2 show 2001). However, further studies (Guo et al., 2010; Luo, 2014) the following characteristics: 1) the crystals are oblique to bedding, showed that the rocks display normal compaction because the with an acute angle of about 40 between bedding and the long overpressured mudrocks exhibited no anomalously low densities axes of crystals, 2) the crystals are sometimes curved, and 3) thrust as evidenced by the poor relationship between mudrock densities faults may occur in the veins with phase 2, indicating shear zones. and effective vertical stress. In addition, the corresponding over- In addition, some of the calcite veins associated with phase 2 pressured reservoir sandstones showed no anomalously high ma- consist of linked en echelon flat-lying veins instead of being hori- trix porosities or geothermal gradients, which relate to the zontally continuous. Bons et al. (2012) also analyzed the sparry characteristics of disequilibrium compaction. calcite formation under shear stress, in which the fibrous sparry Uplift did occasionally exist in these sags as indicated by the calcite crystals were gradually curved. The reason is that the frac- burial history (Qiu et al., 2006 and Fig. 3A, B), thus the horizontal ture formation mechanism influenced the opening trajectory compression effects by uplift on fracture formation are relevant. (Hilgers et al., 2001). Under fluid overpressure, a fracture is formed Here we use the homogenization temperatures of the fluid in- as a vertical opening against gravity, and so the calcite crystals grow clusions and micro-structures of the sparry calcite crystals to steeply to vertically (Rodrigues et al., 2009). Under horizontal determine the tectonic environment during sparry calcite compression, the previously consolidated calcite crystals resist 354 J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359 tectonic shortening whereas the host rock does not. As a result, shear stresses act at the vein margins. In addition, the fractures for calcite vein precipitation are formed by multi-stages of “crack and seal” (Li and Means, 2001; Wiltschko and Morse, 2001). Hence, the calcite veins formed by horizontal compression should clearly be oblique to bedding and sometimes curved. Furthermore, horizontal compression should also result in some thrust faults and some linked en echelon flat-lying veins. Note that the above characteristics rarely occur in the Eocene calcite veins (Figs. 4AeE, 5AeE), indicating tectonic compression rarely controlled the fracture formation in the study area. In the Chinese Eocene studied successions, all the overpressured mudrocks have a vitrinite reflectance (Ro) value of 0.6% or higher, and the zone of strongest overpressuring coincides with the lacustrine source rocks in the central basin, which have a high TOC (>2 wt.%, ranging to 7.3 wt.%) (Guo et al., 2010). Note that a vitrinite reflectance value of 0.6% is the threshold for a thermally mature oil window (Peters et al., 2005), and a 2 wt.% in TOC content is exactly the threshold for sparry calcite formation in the above areas (Figs. 7e9), suggesting oil generation as the dominant overpressure mechanism. Berg and Gangi (1999) calculated formation of oil- generation microfracturing in low permeability mudrocks, in which micro-fractures could form if the sum of hydrostatic pressure and pore-pressure change due to oil generation surpassed the lithostatic pressure. We predicted pressure buildup with depth during oil generation using the method of Berg and Gangi (1999) (see Appendix for calculation of the pressure). Clearly these cal- culations show that the sum of hydrostatic pressure and pore- pressure by oil generation surpasses the lithostatic pressure at about 2500 m depth if the TOC content is 2 wt.%, whereas this sum cannot reach the lithostatic pressure at any depth if the TOC content is 1 wt.% (Fig. 12). The sparry calcite coincidently formed at about 2500 m in the study area (Fig. 3A, B) with 2 wt.% in TOC Fig. 12. Pressure buildup with depth during oil generation for different total organic content as the threshold for sparry calcite formation. Hence, the carbon (TOC) content. DP ¼ pore-pressure change due to the conversion of kerogen modeling matches the described depth and TOC for sparry calcite into oil. formation, suggesting oil generation induced the accommodation space for sparry calcite formation. For the above reasons, we suggest oil-generated overpressure as spaces in the laminated shale are mainly composed of horizontal the dominant mechanism for bedding-parallel fracture formation. fractures with excellent continuity (Figs. 4A, 13A). The pore spaces In addition, the extensional to transtensional deformation of the in the structureless mudstone contain two types: 1) horizontal two basins (e.g., Allen et al., 1998) seldom contributed to the frac- fractures with some continuity (Figs. 4B, 13B), and 2) equant and ture formation, because oblique or vertical fractures associated dispersed pores (Fig. 13C). The vertical stress should be the mini- with calcite veins are rare in the mudrocks (Figs. 4A, 5A, 13A, B). mum stress for horizontal fracture formation, so how can oil- In core and FESEM of the Eocene mudrock samples, the pore generated overpressure make the minimum stress to be horizontal? Von Terzaghi's concepts were traditionally suggested to be responsible for horizontal fracture formation in rocks (e.g., Cosgrove, 1995, 2001). In a purely lithostatic situation, where there are no forces except those of gravity, the greatest effective stress should be vertical (Sibson, 2003). Hence, the Von Terzaghi's concepts require that, for the fractures to be horizontal, the rock must be anisotropic if horizontal compression is not responsible for fracture formation. However, the possibility of horizontal compression is excluded here because the structureless mudstone is not anisotropic (Fig. 5A, B). This suggests that the Von Terzaghi's concepts cannot explain the horizontal fractures formation in the mudrocks of the two Eocene basins. In a recent study, Cobbold and Rodrigues (2007) discovered that the overpressure gradient by fluid overpressure results in upward seepage forces. The seepage forces may become greater than the weight of overburden, causing a tensile stress and consequent elastic stretching. Along with increasing overpressure, the Mohr circles for effective stress become progressively larger in the tensile field. In lithostatic setting, the horizontal stress (sh) is proportional to the vertical stress (sv)(sh ¼kesv, ke ¼ 1/3e1/2; see Cobbold and Rodrigues, Fig. 11. Force analysis of the mudrock under overpressured environment in transtensional to extensional basins. 2007 for explanation). Hence, the least stress is absolute tension J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359 355

and it acts vertically after the seepage force becomes larger than the weight of overburden. Later studies physically modeled fracture formation by mixtures of silica and beeswax microspheres (Lemrabott and Cobbold, 2009; Zanella et al., 2014b). In their models, basal heating caused the beeswax to melt and resulted in vertical seepage forces, which produced horizontal hydraulic fractures, even in a material that was not initially anisotropic or layered. Though bedding-parallel fractures can form in isotropic materials, bedding layers or fabric anisotropy can favor the development of hydraulic fractures parallel to bedding (Cobbold and Rodrigues, 2007). Using this mechanism, the occurrence of bedding-parallel fractures in both laminated shale and structureless mudstone, and the fact that bedding-parallel fractures are not so common and continual in structureless mudstone, can be explained for the Eocene Chinese successions (Fig. 13 A, B). Hence, we suggest seepages forces formed by ﬂuid overpressure induced the accommodation space for bedding-parallel-arranged sparry calcite (Cobbold et al., 2013). The above analysis explained the accommodation space of bedding-parallel-arranged sparry calcite, but the accommodation space for equant and dispersed sparry calcite in structureless mudstones is still unclear. Before reaching the yield point for fracturing, the mudrocks must ﬁrst undergo the initial phase of elastic deformation during progressive loading (e.g., Sulem and Ouffroulth, 2006), enlarging pore size. The isotropic nature of structureless mudstone enables a higher tension strength than that of laminated shale, making for a higher tolerance for a longer period of elastic deformation. Thus, structureless mudstone has many more large pores as compared to laminated shale in an overpressured environment (Fig.13C). In addition, clay minerals are much more prone to elastically deform as compared to other minerals (Potter et al., 2005), thus the pores produced by elastic deformation preferentially occur around clay minerals. Because the minerals are disorganized and chaotic in structureless mudstone (Fig. 5A), the pores formed by elastic deformation should be equant and dispersed (Ochoa et al., 2013). We suggest these pores provide space to accommodate equant and dispersed sparry calcite formation. This mechanism also explains why equant and dispersed sparry calcite crystals are rare in laminated shale due to its relatively low tensional strength.

5.5. Crystal maturation

As the bedding-parallel fractures constitute the main pore spaces in laminated shale, the recrystallized sparry calcite must preferentially form there. In addition, the micrite laminae, as the ion source for the sparry calcite, are also bedding-parallel (Figs. 4B, 14A); thus, the distribution of sparry calcite is also parallel to bedding (Fig. 4CeE). Gonzalez et al. (1992) suggested that the crystals increase in size and number with recrystallization of calcite during diagenesis. As sparry calcite types 1e3 increase in size and number (Fig. 4CeE), they could successively represent three stages of sparry calcite formation (Fig. 14A). The three stages imply that the crystals are laterally discrete in the beginning, then become

Bay Basin. For location of wells, see Fig. 1 (A) 3165.76 m, well F1. Bedding-parallel and continuous micro-fractures form under overpressure in laminated shale (yellow arrow). Width of the micro-fractures reaches tens of micrometers. The image was taken from the blue rectangle in Fig. 4A. (B) 3631.08 m, well L1. Bedding-parallel micro- fractures form under overpressure in structureless mudstone (yellow arrow). The image was taken from the blue rectangle in Fig. 5A. (C) 3631.20 m, well L1. Dispersed pores with equant morphology formed under overpressure in a structureless mudstone (yellow arrow). The image was taken from the red rectangle in Fig. 5A. (For Fig. 13. High-resolution ﬁeld-emission scanning electron microscopy (FESEM) photos interpretation of the references to color in this ﬁgure legend, the reader is referred to of pores and fractures within the mudrocks of the Dongying Sag of the Eocene Bohai the web version of this article.) 356 J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359

Fig. 14. Model of crystal maturation in mudrocks. (A) Three stages of sparry calcite formation in laminated shale. In stage 1, discrete sparry calcite crystals with equant shapes form along micrite laminae. The number increases and sizes of the crystals gradually become larger in stage 2, and so discrete crystals become laterally intergrown. The lateral growth stops but the vertical growth continues, forming fibrous sparry calcite in stage 3. (B) Four stages of sparry calcite formation in structureless mudstone. In stage 1, the sparry calcite crystals are dispersed as equant forms in pores formed by elastic deformation. Stages 2e4 is similar to that of stages 1e3 in laminated shale. However, the horizontal continuity of calcite veins is not as well developed as that in laminated shale. laterally intergrown, and then vertically grow into fibrous forms. 2007). The calcite crystals grow much more rapidly in the direc- However, there are still two questions remaining on the proposed tion of the c-axis in supersaturated solutions because of the high mechanism. Why do the crystals become laterally intergrown charge development (Lahann, 1978). After the crystals become before growing into fibrous forms? What controls the growth laterally intergrown, the crystals with the c-axis exposed to the competition of fibrous forms? The properties of micro-fractures solution grow rapidly, gradually squeezing the space of adjacent answer the first question. The formation of micro-fractures by crystals (Figs. 4E, 14A), illustrating the phenomenon of growth fluid overpressure ranges from several to tens of micrometers in competition. vertical length when not filled (Fig. 13A). This suggests that the In structureless mudstone, the accommodation space formation micro-fractures should vertically grow along with fibrous sparry successively experiences: 1) equant and dispersed pores formation calcite formation because there is not enough accommodation by elastic deformation (Fig. 13C), and 2) horizontal fractures for- space otherwise. Compared with vertical growth, lateral growth in mation after reaching the yield point (Fig. 13B). Correspondingly, the fractures is unhindered because there are no barriers. Hence, the accommodation space formation controls the sparry calcite the crystals become laterally intergrown before growing into formation in structureless mudstones, adding an extra preliminary fibrous forms. The orientation of the c-axes of the calcite crystals stage to the three stages of sparry calcite formation in laminated answers the second question. The fibrous sparry calcite crystals are shales. This preliminary first stage involves pore size enlargement always longer in the direction of the c-axis (Fig. 4F). The c-axis face first by elastic deformation (Fig. 13C), enabling equant and 2þ 2 alternates with a Ca layer and a CO3 layer, while the edge face dispersed sparry calcite crystal growth (Fig. 14B). After reaching the 2þ 2 has equal amounts of Ca and CO3 ions (Moore, 2001). Further- yield point, the elastic deformation stops and fractures begin to more, the sparry calcite minerals grow by precipitation, mainly form (Sulem and Ouffroulth, 2006). Hence, the formation of equant from supersaturated aqueous solutions (Cobbold and Rodrigues, and dispersed sparry calcite stops, whereas the formation of J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359 357 bedding-parallel sparry calcite begins. Since the formation mech- organic-rich (TOC > 2wt.%) mudrock intervals in the Eocene anisms of horizontal fractures in structureless mudstone and Zhanhua and Dongying Sags of the Bohai Bay Basin, and the Biyang laminated shale appear to be the same, the bedding-parallel sparry Sag of the Nanxiang Basin. In laminated shale, sparry calcite con- calcite formation also successively experiences three stages in tains equant and fibrous forms and distribution is parallel to structureless mudstone (Fig. 14B, stage 2e4), similar to the three bedding. However, the sparry calcite crystals can also be dispersed stages of sparry calcite formation in laminated shale (Fig. 14A, stage randomly with equant morphology in structureless mudstone. 1e3). Sparry calcite crystals form in a thermally mature oil window within organic-rich mudrocks. Oil generation induces micrite 6. Implications dissolution, providing the ion source for sparry calcite formation; fluid overpressure induces upward seepage forces, resulting in ac- 6.1. Characteristics of sparry calcite in mudrock commodation space for crystal growth. Before reaching the yield point for fracturing, structureless mudstone could experience a As early as the 1860s, Sorby discovered sparry calcite in period of elastic deformation, enabling equant and dispersed pores mudrocks in southern England (Sorby, 1860). Though sparry calcite for sparry calcite formation. After reaching the yield point, the has been studied for more than a century, this study gives new upward seepage forces result in bedding-parallel fractures for insight into its origins. Firstly, dispersed and random equant sparry sparry calcite growth. Laminated shale, because of its anisotropic calcite is a newly described type of carbonate formed within nature, can be fractured relatively quickly during elastic deforma- mudrock sequences. In previous studies (e.g., Parnell and Carey, tion in comparison to structureless mudstones, showing domi- 1995; Parnell et al., 2000; Rodrigues et al., 2009), fractures were nantly bedding-parallel fractures filled with sparry calcite. Sparry generally suggested to be responsible for sparry calcite formation in calcite formation successively experiences three stages in lami- mudrock, but evidence presented here suggests that elastic defor- nated shale: 1) laterally discrete equant forms, 2) laterally inter- mation is also an important mechanism in structureless mudstone grown equant forms, and 3) laterally intergrown fibrous forms. before reaching the yield point for fracturing. Hence, sparry calcite However, structureless mudstone experiences a period of equant may not always occur in fractures. Secondly, this study also pro- and dispersed sparry calcite formation during elastic deformation vides a mechanism for the growth of fibrous sparry calcite from because of its isotropic nature. This is followed after the yield point equant sparry calcite. This process of fibrous sparry calcite forma- by the development of bedding-parallel fractures for more sparry tion differs from previous interpretations by Means and Li (2001), calcite growth. Fibrous sparry calcite began its growth from equant Hilgers et al. (2001), and Rodrigues et al. (2009). sparry calcite in the Eocene Zhanhua and Dongying Sags of the Bohai Bay Basin, and the Biyang Sag of the Nanxiang Basin because 6.2. The relationship between sparry calcite and source rock of growth competition during the process of fracture opening. Mudrock with bedding-parallel sparry calcite and/or dispersed It has been widely accepted that there is a close relationship and random sparry calcite are generally thermally mature and between calcite formation and organic matter (Gierlowski- organic-rich and may serve as an exploration tool in the search for Kordesch, 2010, p. 25). For example, the formation of coal balls potential source rocks. (e.g., Scott et al., 1996) and carbonate nodules in concretions and cements (e.g., Mitterer and Cunningham, 1985; Curtis and Coleman, 1986; Maliva et al., 2000) were reported to be associated with Acknowledgments degrading organic material in the marine at near surface temperature and pressure. However, the relationship between sparry The China National Key Research Project (2011ZX05009-002, calcite and organic matter at high temperature and pressure was 2016ZX05009-002) supported this study. We are grateful to the not clear. Geological Research Institute in the Shengli and Henan Oilfield for In recent years, the mudrocks with bedding-parallel veins of permission to access the database on these rocks. Jing Wu in the fibrous sparry calcite were commonly found in thermally mature Petroleum Exploration and Production Research Institute, SINOPEC and organic-rich stratigraphic sequences in dozens of worldwide and Wenzhao Zhang in the Research Institute, CNOOC also did locations, including the Posidonia Shale of the NW Germany some helpful work in this study. We also thank Prof. Peter Cobbold (Jochum et al., 1995), the Wessex Basin of Britain (Gallois, 2008; of the University of Rennes and two anonymous reviewers for their Jenkyns and Weedon, 2013), the Neuquen Basin of Argentina revisions and comments. (Rodrigues et al., 2009; Cobbold et al., 2013), and the Magallanes- Austral Basin of Chile and Argentina (Zanella et al., 2014a). A close relationship between bedding-parallel veins of sparry calcite and source rock were noted. This study provides a mechanism for Appendix sparry calcite formation in mudrock, where micrite dissolution and fluid overpressure in the presence of organic matter enable sparry The equations and parameters for fluid overpressure changes calcite formation. Oil generation can be responsible for sparry due to the conversion of kerogen into oil in low permeability calcite formation in bedding-parallel veins in all mudrocks, as well mudrocks. For a detailed mathematical derivation of equations as dispersed and random sparry calcite formation in (7)e(9), see Berg and Gangi (1999). structureless mudstones. The existence of bedding-parallel sparry Time depth variation of kerogen/oil conversion calcite and/or dispersed and random sparry calcite in source rock ð = Þ ð = Þ successions in the Eocene lake basins of east China may be used as a F½ ð ÞzA T exp E RT T0 exp E RT0 fi T t (7) new simple tool to con rm a thermally mature oil window for H 2 þ E=RT 2 þ E=RT0 exploration purposes. F ¼ 1 exp½FðtÞ (8) 7. Conclusions Pore-pressure change due to the conversion of kerogen into Sparry calcite crystals occur in the thermally mature and oil 358 J. Zhang et al. / Marine and Petroleum Geology 71 (2016) 344e359

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