bs_bs_banner
doi: 10.1111/rge.12061
Resource Geology Vol. 65, No. 2: 55–75
Original Article
Depositional and Diagenetic Controls on Pore Structure of Tight Gas Sandstone Reservoirs: Evidence from Lower Cretaceous Bashijiqike Formation in Kelasu Thrust Belts, Kuqa Depression in Tarim Basin of West China Jin Lai, Guiwen Wang, Yu Chai, Ye Ran and Xiaotao Zhang State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China
Abstract Deeply buried Lower Cretaceous Bashijiqike sandstones are important gas exploration targets in the Kelasu thrust belt, Kuqa Depression of Tarim Basin in China. The sandstones are characterized by low porosity, low permeability and strong microscopic heterogeneity due to diagenesis during their geologic history. Mineralogical, petrographic, and geochemical analyses combined with high-pressure mercury injection analysis have been used to investigate the diagenesis, diagenetic minerals, and their impact on reservoir quality. This article addresses the controls exerted by depositional parameters and diagenetic modifications on pore-network characteristics (porosity, pore types, sizes, shapes, and distribution), with the aim to unravel the formation mechanisms of this complex pore structures, and to improve the characterization and classification evaluation for the Bashijiqike sandstone reservoirs. The Bashijiqike sandstones are dominated by lithic arkoses and feldspathic litharenite. The pore system consists of intergranular macropores, intergranular micropores, and intragranular pores. Framework grains are generally heavily compacted. Authigenic quartz, authigenic feldspar, clay minerals and carbonates are the major pore-filling constituents. The pore structure is characterized by small pore radius and poor interconnectivity. Entry pressure reflects the microscopic pore network and macroscopic reservoir property characteristics. Pore structure characteristics are linked to the depositional parameters, type and degree of diagenesis. Clays do not control reservoir pore networks alone, and pores and pore throats are wider in coarser grained sandstones. Entry pressure decreases with the content of the rigid quartz. Compaction and cementation continue to decrease the pore-throat size, while dissolution enlarges pores and pore-throats radius. Considerable amounts of microporosity associated with clay minerals and altered grains contribute to the high entry pressure. Comprehensive Coefficient of Diagenesis (CCD), which considers the integrative effect of diagenesis, shows strong statistical correlations with entry pressure. CCD is an integrative modulus of diagenesis and physical property, and generally the higher the values are, the better the pore structure. It is suitable for quantitatively characterizing pore structure in tight gas sandstones. The results of this work can help assess pore-network characteristics like the Bashijiqike sandstones which had experienced strong diagenetic modifications during their geological history. Keywords: Bashijiqike Formation, diagenesis, pore structure, reservoir quality, sandstones.
Received 3 July 2014. Accepted for publication 2 December 2014. Corresponding author: G. Wang, China University of Petroleum, 18 Fuxue Road, Changping District, Beijing 102249, China. Email:
[email protected]. © 2015 The Society of Resource Geology
55
J. Lai et al.
1. Introduction The Kuqa foreland, which connects the southern Tianshan orogenic belt with the northern part of the Tarim subplate, is a peripheral foreland basin (Jin et al., 2008). The Kuqa depression is a primary gas-producing area in the Tarim Basin (Tang et al., 2014), where several giant gas fields have been discovered, including Kela 2, Dina 2, Yinan 2, Yaha, Dabei and Keshen (Dai et al., 2008; Jin et al., 2008). The main gas-bearing layer is in the Lower Cretaceous Bashijiqike Formation sandstones. Numerous studies have been conducted on the basic geology, stratigraphy, petroleum systems, depositional environments and tectonic evolution for the Kelasu thrust belt. It has been confirmed that the Kuqa foreland basin has good conditions for hydrocarbon generation, migration and accumulation (Shi et al., 2012). Presence of abundant hydrocarbon sources, excellent reservoirs and good seal rocks coupled with advantageous migration pathways and effective traps, are very favorable for the formation of large to middlescale oil and gas fields in the Kelasu thrust belt (Zhang et al., 2011). Despite these encouraged exploration successes, the Bashijiqike sandstones are typical tight gas sandstone reservoirs with high heterogeneity, poor porosity and permeability (Zhu et al., 2012). Exploration and development efforts of the Bashijiqike reservoirs were often challenged by the complicated pore-network characteristics. Knowledge of the size and character of pore throats and pore space in these reservoirs with respect to their potential for producing hydrocarbons is of great importance (Nelson, 2009). Depositional parameters such as grain size, sorting, shape, and matrix content strongly influence subsequent pore-system evolution (Ajdukiewicz & Lander, 2010). Diagenesis takes place at a molecular level (Sadhukhan et al., 2007), it exerts a strong control on the quality and heterogeneity of sandstone reservoirs (Morad et al., 2010). Diagenetic modifications influence the evolution of reservoir spaces and pore structures, pore-throat size and distribution. The complicated microscopic pore structure is mainly interpreted as the result of prolonged and complex diagenetic modifications (Morad et al., 2010). Insight in to the diagenetic processes responsible for the formation and destruction of pore space is vital for the prediction of reservoir quality. It is thus important to quantify the effects of diagenesis on characteristics of pore structures (Okazaki et al., 2014). The impact of depositional parameters and diagenesis on microscopic pore structure of such tight gas
sandstones has been, thus far, poorly explored in the literature. The purpose of this paper is to show how the sandstone composition and texture, and the type and degree of diagenesis, influence the pore-network characteristics, with the aim to unravel the depositional and diagenetic control on the changes in the complex pore network properties during their diagenetic history for the Bashijiqike sandstones, which is critical for future exploration, evaluation and production of reservoirs and has practical applications as well as scientific significance in tight sandstones elsewhere.
2. Geological setting 2.1 Basin evolution The Tarim Basin, located in the southern Xinjiang Uygur Autonomous Region, Western China, is the largest basin in China, with an total area of 560,000 km2 (Zhang & Huang, 2005; Qiu et al., 2012; Gao & Fan, 2014). It is bounded by the Tian Shan to the north, the Kulugetake Shan to the northeast, the Kunlun Shan to the southwest and the Altun Shan to the southeast (Fig. 1; Qiu et al., 2012). It is divided into several tectonic zones, including the Kuqa depression, the Northern depression, the Southwest depression, the Southeastern depression, the Northern uplift, the Central uplift, and the Southern uplift (Fig. 1b; Qiu et al., 2012). The Kuqa Depression located in the northern part of the Tarim Basin covers 28,500 km2, and appears as a nearly east–west elongated structural unit (Fig. 1; Zeng et al., 2010; Ju et al., 2014). The Kuqa depression mainly consists of three structural belts and two sags, which include the Northern Monocline belt, the KelasuYiqikelike structural belts, the Baicheng and Yangxia sags, the Qiulitag structural belt and the Southern Slope belt from north to south (Zeng et al., 2010; Ju et al., 2014; Wu et al., 2014). The Wushi sag and Wensu swells lie on the west of Kuqa depression (Fig. 1c). The Kelasu structural belt is on the second rows of structures in the foothill areas in front of the southern Tianshan Mountains (Jia & Li, 2008). Two major faults occur on the northern and southern flanks of the Kelasu structural belt, and the additional faults parallel to these two main faults are developed in the western and northern flanks of the structure (Xu et al., 2004). The Kuqa depression is a foreland depression formed during the Mesozoic to Cenozoic orogenic events (Zhang & Huang, 2005). The structures in the Kuqa Depression are dominated by thrust faults and related folds (Zeng et al., 2010). The Kuqa depression © 2015 The Society of Resource Geology
56
Pore structure of the Bashijiqike Formation
Fig. 1 (a) Map showing the location of the Tarim Basin within China; (b) Structural subdivision map of the Tarim basin with the Kuqa depression being highlighted; (c) Map showing the structural elements in the Kuche Depression of the Tarim Basin.
has experienced three stages of tectonic movement as a consequence of the northward Indian subcontinent and southward thrusting of the South Tianshan: a peripheral foreland basin stage (from the late Permian to the middle Triassic), an extensional rift basin stage (from the late Triassic to the middle Jurassic) and a rejuvenated foreland basin stage (since the late Tertiary) (Zhang et al., 2011; Ju et al., 2014). Burial history reconstructions were based on the work of Research Institute of Petroleum Exploration and Development of PetroChina Tarim Oilfield Company, which was generated using the software Petromod. The burial and thermal history showing time versus depth and temperature plot for the Bashijiqike sandstones (the Well Keshen 2 as an example) is illustrated in Figure 2.
2.2 Stratigraphy and depositional facies The sedimentary rocks are up to 11,000 m thick in the Kuqa depression, and are mainly terrestrial clastic
deposits (Zou et al., 2006). The Mesozoic and Cenozoic strata are also fully developed in the Kuqa depression (Tang et al., 2014). The Cretaceous in the depression constitutes mainly the Lower Cretaceous, and the Lower Cretaceous is composed of the Kapushaliang Group and the Bashijiqike Formation. The Kapushaliang Group consists of the Yageliemu Formation, Shushanhe Formation and Baxigai Formation from the bottom (Fig. 3). The upper Yageliemu Formation is composed mainly of medium-grained sandstone and fine conglomerate, and it was identified as braided distributary channel deposits of fan delta plain subfacies. The Shushanhe Formation is composed mainly of thin brown or purple red mudstone interbedded frequently with thin brown siltstone and fine-grained sandstone, which was mainly identified as deposits of shallow-lake subfacies or lakeshore subfacies. The Baxigai Formation is composed of brown thick-bedded medium-grained sandstone and pebbly sandstone, which belongs to distributary
© 2015 The Society of Resource Geology
57
J. Lai et al.
Fig. 2 Burial history curve for the well Keshen 2 (E stands for Paleogene in the figure, the well location of Keshen 2 is shown in Fig. 1c).
channel deposits of delta plain subfacies (Wang et al., 2013). During the Early Cretaceous, sedimentation in the Kuqa foreland basin occurred dominantly in an oxic, shallow lacustrine setting, with a relatively arid climatic condition (Jia & Li, 2008; Wang et al., 2013). The depositional environment of the study area is recognized as a fan-braided deltaic prograding setting, where coarse clastic rocks of alluvial fan, fan delta and braided river facies were well developed, with an abundant supply of detritus from the ancestral Tian Shan. The river mouths were frequently blocked with sediments, enabling the rivers to change their channels repeatedly, and deltas were often connected laterally as a result, shaping a delta apron in the mountain front (Jia & Li, 2008).
2.3 Petroleum systems The Kuqa Depression is the main gas supply region for the “Gas Transmission Project” in China (Qin et al., 2007). The main hydrocarbon source rocks are the Upper Triassic Huangshan and Taliqike Formation lacustrine mudstones and thin coal seams and the Lower–Middle Jurassic Qiakemake and Kezilenuer Formation coal beds in the Kaqu Depression (Jin et al., 2008; Zeng et al., 2010). The source rocks contain domi-
nantly type III organic matters based on Rock-Eval pyrolysis and kerogen microscopy studies, and therefore are generally gas prone (Qin et al., 2007; Jin et al., 2008). Deltaic sandstones occurred widely in the Cretaceous, functioning as good reservoir rocks extending over the entire area (Zhang et al., 2011). The thick Tertiary Kumugeliemu and Jidike Formation evaporitic gypsum-bearing mudstones cover the whole area, and provides an abnormal high pressure tight seal, which act as a regional seal for vertical gas migration (Shi et al., 2004; Xu et al., 2004). The Lower Cretaceous Bashijiqike sandstones, together with the Paleocene Kumugeliemu Formation evaporates, constitute a well-developed reservoir-cap rock assemblage in the Kuqa depression (Jin et al., 2008). Anticlines related to faults are the main structural traps in the petroleum systems. Faults communicate with the reservoir but not communicate with the cap rock played important roles for the hydrocarbon migration and accumulation (Zou et al., 2006; Zeng et al., 2010; Zhu et al., 2013). In fact, faulting provides an pathway for gas to vertically charge the reservoirs in the Kuqa depression (Shi et al., 2004). According to fluid inclusion analysis, two periods of hydrocarbon accumulation are determined: the early oil accumulation (24–17 Ma) and the late gas accumulation (5–2 Ma) (Fig. 3) (He et al., 2009; Zeng et al., 2010). Reasonable matches in the timing of trap © 2015 The Society of Resource Geology
58
Pore structure of the Bashijiqike Formation
Fig. 3 Schematic stratigraphy and hydrocarbon source rocks, reservoirs and cap rocks in the Kuqa Depression within the Tarim Basin (modified after Zhao et al., 2005 and Zeng et al., 2010).
© 2015 The Society of Resource Geology
59
J. Lai et al.
formation, fault development and peak gas generation are among the most critical elements that have contributed to the formation of the giant gas fields in the Kelasu thrust belt (Zhao et al., 2005; Jia & Li, 2008).
3. Materials and analytical methods In order to characterize the flow capacity (permeability) and the storage capacity (porosity) of the reservoir, rock properties measurements were performed on 200 samples of 1-inch diameter core plug from nine wells along with 46 samples of 2.5-inch full diameter plug from six wells under the net confining stress of 800, 1980 and 3160 psig respectively. Sixty core samples representative of the Bashijiqike sandstones were analyzed from the Keshen 208, Keshen 2-1-5, Keshen 2-2-4, Keshen 2-2-5, Keshen 2-2-8, Keshen 205, Keshen 206, and Keshen 207 wells. All the samples were selected for thin section epifluorescence, scanning electron microscopy (SEM), and X-ray diffraction (XRD) analyses in The Houston Advanced Technology Center of Core Laboratories. Thin section samples were impregnated with epiflourescent epoxy. The epiflourescent dye in the epoxy helps to detect micropores in the thin section samples. Thin sections were stained for carbonates (calcite, ferroan calcite, and ferroan dolomite) and K-feldspars as an aid for mineral identification. Thin section photo pairs were taken under plain light and UV light from the same field view. Point-count analysis was performed with respect to detrital framework grains, matrix, cement, interstitial minerals, and porosity, as well as the textural modal grain size and sorting parameters using 300 points per sample. Lithology characteristics of the Bashijiqike sandstones were determined by 370 standard thin-sections grinding to a standard 30 μm thickness by the NIKON OPTIHOT-POL optical microscope in the petroleum geology laboratory of PetroChina Exploration and Development Research Institute in China. However, there are also 230 samples prepared by vacuum impregnation with red-dyed resin to facilitate the recognition of porosity in the same laboratory, and they were also stained with Alizarin Red S for identification of carbonate cements. The SEM analysis was conducted on the freshly broken surface of 60 samples to identify pore characteristics and types of pore-filling constituents. Each sample was mounted on an aluminum stub and coated with gold/palladium (Au/Pd) alloy. The regular SEM photomicrographs are secondary electron images.
X-ray Diffraction (XRD) analysis was performed on the 60 samples to obtain semi-quantitative mineralogical data. Samples for whole-rock and clay-fraction analyses are first cleaned of obvious drilling contaminants and then disaggregated in a mortar. The pore structure characteristics (pore, throat size and its distribution) of Bashijiqike sandstones are studied according to the entry pressure, medium pressure, max radius of throat, median radius of throat (R50) and R35 from high-pressure mercury injection capillary analysis (HPMI) of 60 plug core samples. Among these 60 samples, X-ray Diffraction analysis were performed on 40 samples of them, and these 40 samples are polished into casting thin sections. In the HPMI analysis, a mercury porosimeter is used to generate pressure high enough (55,000 psi) to force mercury into all interconnected pores and measure the volume of mercury entered.
4. Results 4.1 Sandstone composition and texture The thin section identification results show that the Bashijiqike sandstones are classified as lithic arkoses, feldspathic litharenites, and less commonly arkoses according to Folk’s classification scheme (Folk, 1980) (Fig. 4). Detrital mineralogy is dominated by quartz (mostly monocrystalline) ranging from 35% to 65%,
Fig. 4 A ternary diagram showing the framework-grain composition of the Bashijiqike sandstones. © 2015 The Society of Resource Geology
60
Pore structure of the Bashijiqike Formation
Fig. 5 Composition and texture of the Bashijiqike sandstones in Kelasu thrust belt. (a) Feldspathic litharenite, medium-grained, subrounded to subangular, well sorted. White grains are mostly quartz (Q) and plagioclase (PF); dark grains are generally rock fragments (RF); yellow stained grains are K-feldspar (KF). Keshen 208, 6602.52 m, Bashijiqike Formation, plane-polarized light (PL). (b) Feldspathic litharenite, moderately sorted, finegrained (0.24 mm), low to moderate sphericity, Keshen 2-1-5, 6725.44 m, Bashijiqike Formation, PL.
A
B
KF
RF PF
PF Q
RF Q
RF
KF RF
feldspar content varies from 17% to 45% consisting of both plagioclase (24.7%) and smaller amount of K-feldspar (7.3%), the lithic rock fragments account for 12–45%, consisting mainly of metamorphic, and volcanic rock fragments, with minor sedimentary rocks fragments. The matrix is dominated by detrital clay minerals generally ranging from 5.7% to 22.2% with an average of 9.1%. The grains range from very fine to medium-grained sands, exhibiting moderate to well sorting and low to moderate sphericity (Fig. 5a, b). Generally the Bashijieike sandstones are a suite of terrestrial clastic rocks with low compositional maturity but moderate petrological textural maturity.
4.2 Porosity, permeability and pore systems Total thin section porosity of the Bashijiqike sandstones, which includes both primary and secondary pores, reveals a range from trace (
