Research On Depositional Facies And Sequence Stratigraphy For The Chang-8 Member Of The Upper Triassic Yanchang Formation, Longdong Region, Ordos Basin

Paleocurrent direction and provenance, sedimentary systems and depositional facies of the Upper Triassic Yanchang Formation from Xifeng Oil Field, Longdong region in the southwestern Odors basin are studied. Temporal and spatial distribution of the depositional facies are revealed, and sequence stratigraphy and its relationship with the source-reservoir-cap assemblages are also discussed in mis paper, based on field work of geological survey and measurements of outcrops, observation and description of cores, analyses of drilling and logging data, light and heavy minerals in sandstones, observation and identification of thin-sections. Many outdated theories and new technological methods related to sedimentology, petroleum geology, lithofacies and paleogeography, geophysical logging are used in the paper.Research on paleocurrent directions and provenances indicate that southwest direction is the primary paleocurrent and northwest is the secondary one, whereas southeast and northeast are the accessory paleocurrent directions. The Ningxia, Liupanshan, Longxi and Qingling ancient lands which are surrounding the southwestern edge of Odors basin, are the dominating provenances for the Chang-8 Member of the Upper Triassic Yanchang Formation, Xifeng Oilfield, Longdong Region. Provenance for the northeast part (Baibao and Yuancheng area) in the study area might come from the Liuliangshan and Yingshan ancient lands located in northeast of Odors basin, which may be the secondary provenance for the Yanchang Formation.Sedimentary systems of the Chang-8 can be divided into alluvial fans, lacustrine-delta and lacustrine facies. Among these facies, lacustrine-delta is domineering hi the study area. The depositional pattern shows that alluvial fans, delta facies and lacustrine facies are developed hi proper order from the southwest to the center of the basin. Depositional patterns of the two sub-oil-bearing beds (Chang-8Jand Chang-82) of the Chang-8 are basically the same.The outcrop of Yanchang Formation hi Ankou-Ruishuihe, suited hi the southwestern edge of the Ordos basin, is composed of Lowstand systems tract, Transgressive systems tract, Highstand systems tract and Regressive systems tract from the bottom to the top, which consists a complete sequence of type I. Top sequence boundary of the Lowstand systems tract is "Lijiapan shale" of Chang-9; top sequence boundary of the Transgressive systems tract is "Zhanjiatan shale" of Chang-7, which presented the first flooding surface (FFS) and the maximum flooding surface (MFS) hi the area, respectively. The Highstand systems tract is bounded withthe MFS in the bottom and with the bottom surface of the Chang-6 as its top boundary. The Transgressive systems tract is composed of delta fades of Chang-6 and Chang-4+5, and fluvial fades of Chang-2+3 and Chang-1. The Chang-8 oil-bearing bed in the study area consists the major part of the Transgressive systems tract, which occurred during the climax phase of the lake development The source-reservoir-cap assemblages and distribution of hydrocarbons of the Yanchang Formation are controlled both by depositional environment and by characteristics of the sequence stratigraphical patterns of the Yanchang Formation. For the Chang-8 oil-bearing bed, there are two types of source-reservoir-cap assemblages: one is with the source rocks hi the overlain beds and reservoirs in its lower part, and another is the lateral reservoirs with the lateral source rocks hi the beds. The deep lake mudstones and shales of the Chang-7 overlain directly on the Chang-8, and mudstones in deep waters of the Chang-8 depositional environment offered sufficient hydrocarbons for Chang-8, which are also excellent caps. Mat-like sandstones in delta-front fades of the Chang-8 Member are the major hydrocarbon reservoirs.