Character of Terrestrial Sequence Stratigraphy and Depositional System in Incised Valley, Outcrop Area of Karamay Oilfield, Junggar Basin, China

In the Karamay oilfield located on the northwestern margin of Junggar basin, Xinjiang, China, a large area of the Karamay Formation is exposed at outcrop in the northeast of the oilfield, a consequence of thrusting. The Middle Triassic Karamay Formation in the outcrop area is a type of terrestrial third-order sequence, bounded by two easily recognizable sequence boundaries: a regional surface of angular unconformity (SB1) at the base and a regional unconformity (SB2) at the top. Within the Karamay Formation, two lacustrine expansion events can be recognized and be used to identify both the initial and the maximum lacustrine flooding surfaces. The two lacustrine flooding surfaces serve as references for the classification of this third-order sequence-Karamay Formation into the following three sedimentary successions: a lower lowstand systems tract (LST), a middle lacustrine-expanding systems tract (EST), and an upper highstand systems tract (HST). Different systems tracts are composed of different depositional system assemblages. In this paper, each depositional system is described in detail. The lowstand systems tract in the study area is characterized by incised valleys. At the base and on the margin of the incised valleys occur alluvial fan depositional systems, and in the upper and distal parts of the alluvial fan, low-sinuosity river depositional systems. The lacustrine-expanding systems tract consists of a lacustrine depositional system and a lacustrine delta depositional system, overlying the lower incised valley fills. The highstand systems tract is filled by a widespread lacustrine braided delta depositional system. The analysis of sequence stratigraphy in this paper serves the description of the spatial distribution of the reservoir. The depositional system analysis serves the description of the reservoir types. Field investigations of oil sandstone and oil seepage show that the Karamay Formation is composed of several types of reservoirs. However, two types of high quality reservoir occur both in the upper interval of the lowstand systems tract and in the lacustrine-expanding systems tract: gravelly low-sinuosity channel in the distal fans and sandy-gravelly distributary channel in the lacustrine delta plain.

Sedimentary features and sequence stratigraphy of the successions around the Carboniferous-Permian boundary in the Ordos Basin:links to glacial and volcanic impacts

The sedimentary successions around the Carboniferous-Permian boundary(CPB)in the Ordos Basin were investigated using extensive outcrop,borehole,well logging,thin section,and geochemistry data to study sedimentary and sequence stratigraphic responses to glaciation and volcanism in paleotropical transitional strata.Within the studied interval,five distinct lithofacies have been identified,including bauxite,coal,and carbonaceous shale(No.8+9 coal seams),sandstone(Qiaotou),limestone(Baode),and mudstone,which can be classified into three lithofacies associations.The most complete lithofacies associ-ation is composed of bauxite,coal,carbonaceous shale,sandstone,limestone/mudstone or their combinations from the bottom to the top,while coal and carbonaceous shale,as well as sandstone,are absent locally,resulting in the formation of the other two types of lithofacies associations.The occurrence of bauxite in-dicates shelf exposure and weathering,the occurrence of coal and carbonaceous shale indicates swampiness of the shelf,and the occurrence of sandstone reveals river rejuvenation;all of these are thought to be sedimentary responses to the transcontinental glacier expansion in Gondwana around the CPB.The presence of limestone and mudstone indicates carbonate platform and lagoon deposition,respectively,in the context of the earliest Asselian transgression caused by volcanism-induced glacier melting.The lithofacies associations record the regressive-transgressive cycles that occurred because of glaciation and volcanism near the CPB.The top surface of bauxite can be used as a sequence boundary,while the lowstand systems tract consists of the No.8+9 coal seams and the Qiaotou sandstone,and the transgressive systems tract consists of the overlying Baode limestone and laterally equivalent mudstone.The lowstand systems tract,which contains source rock and hydrocarbon reservoirs,and the overlying transgressive systems tract,which serves as cap rock,form an excellent source-reservoir-seal combination.