Characteristics of silty laminae in Zhangjiatan Shale of southeastern Ordos Basin, China: Implications for shale gas formation

Abstract

Shale oil and gas have been discovered in the lacustrine organicrich Zhangjiatan Shale of the Upper Triassic Yanchang Formation, Ordos Basin, China. Core observations indicate abundant silty laminae in the producing shales. This study documents the stratigraphic distribution of silty laminae and their relationship with interlaminated clay laminae. The type, structure, and characteristics of pores and mineral composition of silty laminae were observed and analyzed through thin section and scanning electron microscopy, X-ray diffraction, low-pressure CO 2 and N 2 adsorption, mercury porosimetry, and helium pycnometry. Results from silty laminae are compared with those of clayey laminae. The frequency and thickness of silty laminae vary over a wide range. The thickness ranges from 0.2 to 4 mm and is 1.5 mm on average; the frequency ranges from 4 to 32 laminae/m and is 23 laminae/m on average. The thickness percentage of silty laminae in the measured segments ranges from 6% to 17%. Silty laminae consist of quartz, feldspar, mixed-layer montmorillonite, and chlorite. In comparison to clayey laminae, non-clay detrital grains are larger, quartz and feldspar are more common, and clay minerals are less abundant. Pores in silty laminae are primary interparticle, dissolutional, intercrystalline, and microfracture types. Mesopores (2-50 nm in diameter) and macropores (50 nm-1 μm) are common, whereas, micropores ð<2 nmÞ are rare; the distribution of pore diameters is multimodal. However, microscopic pores with a diameter commonly smaller than 100 nm are common in

Investigation of Fracturing Network Propagation in Random Naturally Fractured and Laminated Block Experiments

“…These Yanchang shales have recently been selected as target plays for shale gas exploration in typical terrestrial strata in China. All members consist of shales, mudstones and sandstones, among which Member Chang 7 of the Yanchang Formation is composed of shales predominantly interbedded with silt sandstones, which is characterized with typical laminae or interbed structures [8][9][10][11][12]. The silty laminae structure in the continental shale plays in the Ordos Basin in China leading to the extraction of shale gas is significantly different from those of marine shales documented in the USA and elsewhere [2,6].…”

“…For typical silty laminae structural shale formations, studies were mainly focused on the occurrence of silty laminae [16], depositional environments [17], hydrodynamic conditions [18], origin of silty laminae [19], the shale gas generation and distribution [20]. From the exploration and experimental results of the scholars [12,21], it is obvious that silty laminae forms significant migration conduits and contribute significantly to reservoir storage. How to maximize the total stimulated reservoir volume plays a major role in successful economic gas production for shale gas.…”

Origin and depositional model of deep-water lacustrine sandstone deposits in the 7th and 6th members of the Yanchang Formation (Late Triassic), Binchang area, Ordos Basin, China

“…Member 7 was deposited during an episode of accelerated subsidence, when the water depth increased from 50 to 120 m, causing significant lake expansion. Laterally extensive black shale was deposited during this time (Lei et al 2015). Deep-water sandstones of Member 7 were deposited on top of the black shale.…”

“…The 7th and 6th members of the Yanchang Formation in the Binchang area of the Ordos Basin were deposited in a deep-water environment (Lei et al 2015) and are the main production formation of tight oil in China (Yang et al 2012;Ding et al 2013;Yuan et al 2015). Understanding the origin, distribution pattern, scale and continuity of this sandstone is important, and these factors must be considered during exploration and decision-making in the development of the oil field.…”

A Laboratory Study of the Effects of Interbeds on Hydraulic Fracture Propagation in Shale Formation

“…In addition, the shales discovered in the Ordos Basin, China, are generally characterized by silty laminae and interbeds, and a mass of natural gas also exists in silty laminae and interbeds [17,18]. At present, the research on this kind of shales is mainly focused on the gas production, physical and chemical properties of the shales with interbeds [19][20][21]. Therefore, it is necessary and significant to study the effects of laminae or interbeds on fracture propagation.…”