The Junggar Basin is a typical superimposed system characterized by multilateral steep thrusts and extrusions and one of the primary petroliferous basins in Western China. To date, geologists have been most interested in the basin's tectonic evolution and petroleum geology characteristics. Based on field outcrops, lithologic combinations, logging curves, and seismic reflection characteristics, we present a systematic study of the sequence stratigraphy pattern for the Jurassic central Junggar Basin. Herein, we identify the central Junggar area as having a “dual” characteristic sequence framework composed of a lowstand system tract (LST) and a transgressive system tract (TST), which is distinguishable from the traditional three system tract (LST, TST, and highstand system tract [HST]). In each third‐order sequence, the LST consists of conglomerate or pebbly sandstone deposits, displaying the characteristics of braided channel and fan delta. The TST consists of siltstone, argillaceous siltstone, and mudstone, displaying fine‐grained, shore‐shallow lacustrine deposits. It is considered that the combination of multiphasic thrust extrusion and subsequent, steady deposition is likely responsible for the dual‐system tract. The forming process of the dual‐system tract pattern could divide into four stages: the quiet stage of tectonic activity, the early stage of compression, the continuous stage of compression, and the rapid rebound stage. Additionally, two reservoir predictive models are proposed: the progradational predicting model on the edge of the depression and truncated and pinchout predicting model on the palaeo‐uplift. We conclude that the coarse clastic unit of the LST has bright prospects for oil and gas exploration within the superimposed system in Western China.
In the last several years, the massive success of the Fuling shale gas field in south‐western China has shown great potential in finding giant shale gas fields in the Upper Yangtze area, and it has also attracted the increasing attention from geologists to find more shale gas reservoirs around the world. However, there is still a great dispute as to whether the neighbouring Lower Yangtze area is a next potential shale gas block in China. In this study, we evaluate preliminarily the shale gas potentials of five important sets of marine shales based on systematic analyses for the sedimentary environment, thickness, total organic carbon (TOC), thermal maturity, and other organic geochemistry parameters of both geological records from outcrops and wells in the Lower Yangtze area. The first two sets of marine shales occur in the Lower Cambrian and the Ordovician, and they all are of over–high organic maturity due to strong effects from regional tectonic movements and thus have poor potential. Although the Lower Silurian shales are widely distributed in the Lower Yangtze region, a terrigenous delta, a low amount of organic matter (TOC < 1.2%), and late tectonic activity together created unfavourable conditions for the generation and accumulation of the Lower Silurian shale gas. The Permian black shales occur in the Middle and Upper Permian. Several proxies, such as the appropriate maturity (vitrinite reflectance Ro of approximately 2.0%), abundant organic matter (TOC of approximately 4 wt%), good total hydrocarbon potential (S1 + S2), and high level of hydrocarbon conversion have resulted in good conditions for shale gas generation. However, the relative thinness (only 30 m), severe tectonic modification, and regional cap rock are not favourable for shale gas preservation. The Lower Yangtze area therefore is not an excellent block for shale gas exploration compared to its neighbouring Upper Yangtze area. However, in some areas with less structural deformation, the possibility of finding shale gas reservoirs cannot be ruled out, especially for the Permian shales, which had a greater opportunity to generate and accumulate shale gas than the other marine shales. In addition, for new shale gas exploration breakthroughs in the future, structural studies are essential in offering better understanding of the preservation conditions in the Lower Yangtze area.
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