Marine and Petroleum Geology 71 (2016) 344e359 Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 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 gen- eration, 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) hor- izontally 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.