The Yangye Formation reservoirs have various types of pores and reservoir spaces, with inorganic pores as the major pores, and the types include intergranular pores, intragranular pores, dissolution pores and moldic pores, with intergranular pores and intragranular pores as the major pores. There are few organic pores, and the diameters of the pores under a microscope mostly range from 0.2 to 5 µm (Fig. 5). The intergranular pores are mainly formed by the random arrangement of flaky clay minerals and small amounts of feldspar, quartz and calcite particles, with organic debris interspersed locally, and the particles support each other to form a relatively stable structure. The pore shapes are mostly polygonal, rectangular and irregular shapes. Intragranular pores mainly develop inside granular, plate-columnar, and sheet-like minerals. In particular, during diagenesis, clay minerals easily dehydrate and transform, and their volume decreases to form secondary intragranular pores [23–25], which are oval, triangular, rhombic, polygonal and irregular under a microscope. Simultaneously, the feldspar and carbonate minerals in shale are prone to corrode and form circular and polygonal dissolution pores. In general, the Yangye Formation reservoirs have developed various inorganic mineral pores and thus have good reservoir properties, and all pores can serve as good oil and gas reservoir spaces.
4.3 Hydrocarbon-bearing characteristics of the shale section
Well TNY1 is a shale gas geological survey well with cores obtained from the whole shale section. The tectonic location of the Well TNY1 is the northern part of the Selayin monocline in the Southwestern Tarim Basin, and the main target layer is the shale in the upper member of the Yangye Formation. Overall, the upper member of the Yangye Formation in the Well TNY1 is characterized by “the upper high-angle fractures containing oil and the lower shale containing gas”, and oil-bearing cores of different degrees were observed below 171 m. A total of oil display sections of 12.45 m /58 sections are obtained via geological logging. The development of fractures in the oil core is mainly vertical fractures, mostly small fractures, followed by middle fractures. The filling material of the fractures is mostly calcite, and the filling degree is mostly semi filled, with few pores. The oil-bearing lithology is generally grey fine sandstone-grey black siltstone. The oil traces are dominant, with occasional oil spots and oil stains. The types of reservoirs are high-angle fractures and dissolution pores, and very few matrices contain oil. The fluorescence is mostly milky white, light blue, or light yellow, and the crude oil is brown-colourless (Fig. 6).
Good gas-bearing cores are found in the Well TNY1 below 310 m. The gas logging has been divided into 30 abnormal gas logging sections, with a cumulative thickness of 88 m. The background total hydrocarbon (TG) ranges from 0.028–2.728%. The maximum total hydrocarbon ranges from 0.14–14.27%, and the ratio of the maximum value to the background value is 2.02–9.91. The main component is methane, with a methane content of 0.06%-10.21%, followed by ethane, with a ethane content of 0.0083%-0.89%. Bubbles generally appear when the core is immersed in water (Fig. 7). When the tight shale core is immersed in water, the small diameter (0.2–0.5 mm), long duration, dense bead-shaped bubbles appear, with a long duration of approximately 72 hours. When the sandstone interlayer core is immersed in water, interrupted exuding bubbles with large diameters (1–3 mm) and short durations appear. In particular, for the well sections dominated by shale with thin sandstone interlayers, dense, energetic, and persistent bubbles appear, with a high gas content. The gases collected by desorption can be ignited, and the flame is blue-light yellow.
The gas sample test results show that the total gas content ranges from 0.07 to 1.29 m3/t, with an average value of 0.41 m3/t; the gas loss ranges from 0.04 to 0.93 m3/t, with an average value of 0.22 m3/t; the gas content of the desorbed gas ranges from 0.01 to 0.77 m3/t, with an average value of 0.16 m3/t; and the residual gas content ranges from 0 to 0.15 m3/t, with an average value of 0.03 m3/t. In terms of the gas content, loss gas accounts for approximately 54%, desorbed gas accounts for approximately 39%, and residual gas accounts for approximately 7%. The total gas content of the 6 desorption samples is ≥ 1.0 m3/t, and the maximum is 1.29 m3/t. The thickness of the corresponding gas shale section is 73.69 m, with a total of 3 gas shale layers, and the methane content of the desorption gas ranges from 52.16 to 88.42%.
4.4 Distribution characteristics of shale gas in the Yangye Formation
The organic-rich shale of the Yangye Formation is widely distributed in the Selayin monoclinic zone at the Yigeziya Township-Tamu area and the Aijiekeagezi-Kusilafu complex syncline core in the Jurassic intermontane basin. The Jurassic overburden of the Selayin Monocline consists of continuous sediments from the Cretaceous to the Neogene, with surface outcrops distributed in bands along the NNW-SSE direction and dipping into the Tarim Basin from west to east at an average inclination of 45°; the burial depth gradually increases, with the deepest part reaching 3,700 m. The dip angle gradually decreases to 10–20° from shallow to deep, and the thickness of source rocks gradually decreases from west to east (Fig. 8). The source rocks of the Yangye Formation in the Jurassic intermontane basin are generally distributed in a N-SW-E direction along the Keyizi-Kusilafu area and are most common in the eastern Aolikedong area of the syncline core, and the thickness decreases from the syncline core towards the surrounding areas. At present, drilling projects have confirmed the gas-bearing nature of the Yangye Formation shale in the two tectonic units of the “Selayin monocline” and the “Jurassic intermountain basin”, the distribution of high-quality shale is significantly influenced by the depositional environment and distribution of sedimentary phases of the entire Southwestern Tarim Basin area, and later tectonic modification events also constrain the shale gas preservation conditions.
Regionally, the entire Southwestern Tarim Basin area has fully entered the continental basin facies sedimentary period since the Jurassic, the sediments are mostly distributed in the divided faulted depressions and sags in the piedmont of the Kunlun Mountains and the piedmont of the South Tianshan Mountains, and the distribution area is relatively small. The Jurassic sediments are mostly distributed in bands along the piedmont of the Kunlun Mountains, which is related to the morphology of the sedimentary basins. Within the independent sedimentary area, Jurassic sediments are distributed in a ring shape, with coastal, shallow lake facies, and swamp facies at the centre and fluvial and alluvial fan facies outwards [26–29]. The southwestern area of the Tarim Basin experiences a stable sedimentary period, and the area of the lake basin further expands. Due to the anoxic environment of the deep water body, a set of interbedded strata of dark shale and shale are deposited. Three semideep lake-deep lake subfacies sedimentary areas centred on the Yigeziya-Tongyouluke-Qimugan, Kayizi-Aolike-Kusilafu and Takela are formed in the piedmont zone of the Southwestern Tarim Basin area, the delta plain-front subfacies develop at the periphery, and the range of deep-water sedimentary facies is relatively large, resulting in the development of lacustrine dark mudstone in the Southwestern Tarim Basin area (Fig. 9).
The areas with high TOC contents in the source rocks of the Yangye Formation are distributed in the Selayin monoclinic zone in the north (Yigziya cross section-Tamu area) and the Kusilafu area in the southern Jurassic intermountain basin in the central part of the study area (Fig. 10). The TOC content of the samples near the piedmont exceeds 2% on average, showing a trend of decreasing TOC content from west to east, from the piedmont zone to the basin (Fig. 10). In the Yangye Formation outcrop area of the intermountain basin, the TOC content tends to be low in the central part and high in the northern and southern areas, and the average TOC content is up to 2.31% in the southern part of the intermountain basin outcrops near the Kusilafu area (Fig. 10). The areas with a high degree of thermal evolution are mainly distributed around the Aolike area of the Jurassic piedmont basin, and the maturity of source rocks gradually decreases eastwards (Fig. 11). A comparison revealed that the distribution characteristics of high-quality source rocks in the Yangye Formation are closely related to the sedimentary facies distribution. In general, the thickness of shale, the abundance of organic matter and the evolution level of shale in areas near deep water are generally greater than those in areas with shallow sedimentary water.