Favorable prospects for the exploration of shale gas have been demonstrated in the Ordos Basin, China. Outcrop and core observations indicate that there are abundant laminas in the shale strata, which exert a great influence on hydro-fracture propagation, gas storage and fluid flow. In this study, the continental shale of the Chang 72 Member, collected from the south of Ordos Basin, was investigated to characterize the geometric heterogeneity. Laminas at multiple scales were observed and measured using conventional logging, borehole TV, core analysis, scanning electron microscopy, and the Particle and Crack Analysis System.
Lihui Li1,2, Beixiu Huang1,2, Yufang Tan1,2, Xiaolong Deng1,2, Yanyan Li1,2 & Hu Zheng3
These measurement tools correspond to the meter scale, decimeter scale, centimeter scale, millimeter scale and ten-micrometer scale, respectively, with measured thicknesses of 2.26 m, 2.09 dm, 1.70 cm, 1.48 mm and 11.70 μm, respectively. Fractal theory was used to analyze the power exponent distribution of the lamina thickness, with a resulting fractal dimension of 1.06. Finally, a geometric heterogeneity model was proposed for the Upper Triassic Yanchang Formation in the study area and verified by a modeled thickness of 26.30 m for the Chang 72 Member at the 10-m scale. The model facilitates cross-scale analysis and provides parameter guidance for heterogeneity characterization in the numerical simulation and model test of the shale gas reservoir.
Recently, inspired by the successful exploration and commercial production of shale gas in the USA, many countries and regions have conducted investigations to evaluate shale gas production1,2,3,4. Organic-rich shale is deposited in both marine environments and non-marine environments: marine-continental transitional environments and continental environments5, 6. The typical shale formations in North America are mainly deposited in marine environments: the Barnett Shale in the Fort Worth Basin3, 7, the Eagle Ford Shale in the West Texas Basin8, 9, the Haynesville Shale in the North Louisiana Salt Basin7, 10, the Marcellus Shale in the Appalachian Basin10,11,12, and the Fayetteville Shale9, 13 and the Woodford Shale in the Arkoma Basin10, 14. A minority of oil shale is formed in lacustrine environments, such as the Green River Formation of Colorado and Utah15 and the Wilkins Peak Member of the Green River Shale in Wyoming16,17,18. By contrast, the marine organic shale in China accounts for only 1/3 of all recoverable shale gas resources, such as the Longmaxi Formation and Qiongzhusi Formation in the Sichuan Basin5, 6. Moreover, approximately 2/3 of this shale gas is discovered in marine-continental transitional facies and continental facies19, for instance, the Xujiahe Formation in the Sichuan Basin and the Shanxi Formation and Yanchang Formation in the Ordos Basin20. To date, preliminary investigations on the continental shale of the Ordos Basin have estimated the reserves of gas to be 677 × 108 m³ 21, indicating a favorable geological condition for shale gas accumulation and exploration.
Similar to a conventional sandstone reservoir, a shale reservoir is heterogeneous22. Influenced by the local climate, the deposition, diagenesis and tectonization in forming the shale reservoir varies, leading to its inhomogeneous characteristics in the spatial distribution and internal properties23, 24. Due to the complex and substantial tectonic movements, the shale gas reservoirs in China are characterized by strong deformation and erosion, well-developed fractures and faults, and high thermal maturity and considerable heterogeneity, in particular continental shale. In the shale fracturing process, heterogeneity is one of the main challenges influencing the prediction of productivity23,24,25,26,27,28,29. Based on geochemistry, petrology and sedimentology analyses, the vertical heterogeneity of the Lower Silurian Longmaxi marine shale can be suggested by its lithofacies, graptolite species and abundance, mineralogy, sedimentary structure, fracture, total organic carbon (TOC) and gas content28.
In continental shale reservoirs, the heterogeneity is mainly characterized by the sharp variation of the TOC content, capability of hydrocarbon generation and expulsion, pore structure and mechanical parameters, as well as high density and frequency of developed laminas22. Studies of the interbedded layers of shale suggest that sandy laminas are favorable sites for oil and gas to accumulate and for fractures to propagate, providing a migration passage for gas flow30, 31. Regarding the continental shale reservoir of the Ordos Basin, studies have shown that as more laminas develop, more free gas and solution gas are contained, resulting in less absorbed gas32. This result indicates that the development of laminas has an important effect on gas production. Unfortunately, reports on the geometric heterogeneity of the continental shale in the Yanchang Formation are sparse.
Numerical simulations and models are commonly utilized for investigating the hydraulic fracture behaviors of shale formations32,33,34,35,36,37,38,39,40,41,42,43,44. Aiming at probing the influence of reservoir heterogeneity on hydro-fractures and optimizing the fracturing design, numerical simulations were performed to simulate the fracture network propagation45. The effects of the laminar structure on the hydraulic fracture propagation in shale were also investigated through laboratory models46. The multi-layer simulations indicate that there is a distinct retardation in fracture propagation passing the interface of layers with different Young moduli47, and a heterogeneous reservoir with multiple layers has a pronounced impact on the fracture geometry and ultimately the production performance45,46,47. It should be noted that in the majority of models, the material heterogeneity used to be characterized with the hypothesis that the properties (failure strength and elastic modulus) of rocks are randomly distributed following a Weibull law47,48,49,50,51. However, a gap exists between the randomly generated geological structures in the simulated model and the real geological structures of the reservoir, and it would be more precise to design a model with the in-situ geological structures, including the lithology, real layer thickness, and attitude. To date, few studies on the stratified layer thickness of shale targeted for hydraulic fracturing, as well as on the characterization of laminas at multiple scales, either macroscopic scales or microscopic scales, are available for reference.
In this paper, the geometric heterogeneity of a continental shale gas reservoir is presented through a case study of shale in the Chang 72 Member, Ordos Basin of China. Conventional well logging, digital borehole TV, core observation, SEM scanning and PCAS analysis were performed to characterize the lamina composition and layer thickness at multiple scales. Fractal theory was used to define the geometric heterogeneity from the macroscopic scales to the microscopic scales. The study of the geometric characteristics provides a reliable reference for parameter selection in the simulation and modeling of shale reservoirs, especially for the trans-scale analysis in the Southern Ordos Basin, China.
The Ordos Basin is located in the western part of the North China Craton, covering an area of approximately 32 × 104 km2. It is a polycyclic superposition basin with vast oil and gas reserves52, 53, forming a large asymmetrical syncline with a broad, gently dipping eastern limb and a narrow, steeply dipping western limb. According to the structural evolution and present morphology, the basin is divided into six major structural units: the Yimeng uplift in the north, the Weibei uplift in the south, the central Yishan slope, the Jinxi flexural fold belt in the east, the Tianhuan depression, and the western edge thrust belt in the west (see Fig. 1).
Figure 1 Ordos Basin structure and location of study area. (A) Map showing the Ordos Basin in China. (B) Tectonic map of Ordos Basin, modified after Lei (2015)32; the study area in the south part of the Weibei uplift is outlined by a red box. The source of the map is the Geological Society of America bulletin, Vol. 99(4), April, 2015. Permission for the map was obtained from an Open Access license with license number 4026530233979. The figure was created using the Chinese version of CorelDraw Graphic Suite X8. (http://www.coreldraw.com/cn).
The basement of the Ordos Basin, formed in the Archean and Paleoproterozoic, has experienced five evolutionary stages: the aulacogen of the Meso-Neoproterozoic, the shallow marine platform of the Early Paleozoic, the stand plain of the Late Paleozoic, the inland depression of the Mesozoic, and the fault depression of the Cenozoic32. Due to the Late Triassic tectogenesis, a large-scale inland freshwater lake was formed in the internal part of this basin, in which the Yanchang Formation was deposited. Subsequent tectonic activities from the late Triassic to the early Cretaceous profoundly affected the generation, migration and accumulation of hydrocarbons. The main source units of this basin is the Upper Paleozoic strata represented by the Carboniferous-Permian coal seams52 and the continental shale in the Yanchang Formation of the Upper Triassic strata53.
Based on the evolution of inland lake in the late Triassic, the Yanchang Formation can be divided into 10 Members (Fig. 2). The lowest Chang 10 Member was deposited in the initial stage of the lake and overlaid by the Chang 9 and 8 Members during the period of major transgression when thermal subsidence occurred. The Chang 7 Member was formed with rapid subsidence, and the water depth reached 50~120 m at the peak stage of significant lake expansion54. The Chang 4~6 Members marked an episode of constructive deltaic infilling with a decreasing subsidence rate.
Figure 2 Stratigraphic column of the Yanchang Formation and its sub-members (modified after Guo (2014)4). The source of the map is Marine and Petroleum Geology, Vol. 57, May, 2015. Permission for the map was obtained from an Open Access license with license number 4103990262253. The figure was created using the Chinese version of CorelDraw Graphic Suite X8. (http://www.coreldraw.com/cn) 57: 509–520.
The Chang 1~3 Members were mainly deposited during a period of major contraction, and the lake disappeared gradually with the deposition of the Chang 1 Member, dominated by a swamp environment. The Chang 9 and 7 Members have long been recognized as the main high-quality source rock and the key strata with abundant shale gas. During the depositional stage of the Chang 7 Member, the tectonic movements were relatively active due to the Indosinian tectonic movement in the northwestern area of the basin, causing obvious regularity in the changing of the sedimentary facies. According to the cycle of sedimentation, the Chang 7 Member can be subdivided into three members: Member 71, Member 72 and Member 73. During the episode of the Chang 73 Member deposition, the lake reached its largest area, with less developed turbidite sandstone of semi-deep and deep lacustrine facies. During the depositional period of the Chang 72 Member, the area of the semi-deep and deep lacustrine facies decreased dramatically, with a relatively well developed delta-front sand-body and turbidite sandstone in the phase of the semi-deep and deep lake. In the last Member 71 stage, the lake decreased further, leading to the well development of the delta-front sand-body and turbidite sandstone of the semi-deep and deep lake facies55.
Figure 3 Oil shale thickness distribution of the Yanchang Formation in southeast Ordos Basin, The figure was created using the Chinese version of CorelDraw Graphic Suite X8. (http://www.coreldraw.com/cn).