162 / 2019-06-03 09:04:44

Development of depressed basin sublacustrine fans and significance to petroleum exploration-A case study of the Chang 7 Member of the Yanchang Formation, southeastern Ordos Basin, North China

sublacustrine fan; Chang 7 Member; Yanchang Formation; Ordos Basin

The Chang 7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin represents a typical deep-water depositional sequence. Presently, over 100 million tons of proven reserves are reported in the Xin'anbian area of the Western Ordos Basin, suggesting significant exploration potential. However, the Chang 7 reservoir is characterized by sporadic discoveries in the southeast around Zhidan and Ganquan. On the basis of field outcrops and core observations, light and heavy mineral provenance analysis, petrological studies, sedimentary facies analysis, well log data, and particle size analysis, we evaluated the characteristics of deep-water gravity flow deposition in the Chang 7 Member of the Yanchang Formation in the southeastern Ordos Basin. It is attributed to gravity flow deposition in the background of semi-deep and deep lake. Provenance is largely attributed to areas in the northeast, with subordinate contribution from areas in the south. Multi-stage gravity flows deposited several sublacustrine fans in the center of the basin. The turbidite fans are developed in the background of gentle slope without any erosional canyon between the fan and delta front. The turbidite fan is lobed and tongue shaped in planar view, with the grain size decreases away from the source. The sublacustrine fan includes an inner, a middle and an outer fan. Blurred boundary between inner fan and middle fan while outer fan is easy to identify. The middle fan is the most developed and contains branch channel and interchannel microfacies. The widely distributed branch channel is an important reservoir facies of the Chang 7 Member reservoirs. Thick, widely distribute, good quality source rocks (TOC = 2–6%) have attained the peak stage of oil generation (Ro = 0.9–1.2%). The superimposition of the sublacustrine fan sand bodies and the wide distribution of good quality source rocks favor the formation of large lithologic reservoirs characterized by source–reservoir integration, self-generation and self-storage, and near-source accumulation.