Abstract
The Chang 7 interval of the Upper Triassic Yanchang Formation in the Ordos Basin represents a typical deep lacustrine depositional sequence. On the basis of field outcrops, cores, well logs, light/heavy mineral provenance analysis, and petrological studies, we evaluated the characteristics of deep-water gravity flow deposition of the Chang 7 interval and constructed a depositional model. The sediments mainly came from the northeast of the study area, and multiple sublacustrine fans were deposited in the center of the basin. Different from the deep-marine fan, the sublacustrine fan in the study area develops under the background of gentle slope without any erosional canyon between the fan and delta front. Gravity flow deposits in the study area can categorised into three groups: sand debris flow deposits, turbidity current deposits, and deep-water mudstone deposits. The main channel and branch channel are mainly developed with thick massive sandy debris sandstone, while the channel lateral margin and branch channel lateral margin are mainly developed with middle massive sandy debris sandstones and turbidite sandstones, which from bottom to top, the thickness of sand layer becomes thinner and the grain size becomes smaller. Thin mudstone is developed between channels; the lobe fringe includes sheet-like turbidite sandstones and deep lake mudstones. The widely distribute, good quality source rocks ( TOC = 2 % – 6 % ) developed in deep lacustrine have attained the peak stage of oil generation ( R o = 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.
Highlights
The continuous development of new technology and advances in knowledge on oil and gas exploration and development have remarkably improved exploration and development in deep-water basins worldwide
The results reveal the characteristics, the sublacustrine fan development, and the significance of the gravity flow sediments for exploration in the Chang 7 interval in the southeastern Ordos Basin
The outcropping sediments deposited during the Late Triassic, which were described in detail to illustrate the vertical characteristics of gravity flow deposits and interpret lithofacies
Summary
The continuous development of new technology and advances in knowledge on oil and gas exploration and development have remarkably improved exploration and development in deep-water basins worldwide. Major discoveries of oil and gas in tight sandstones include the Gulf of Mexico, the North Sea, and the Norwegian Sea [1–10] These discoveries prompted deep-water exploration in marine basins such as the Qiongdongnan and the Yinggehai and continental basins such as the Ordos and the Bohai Bay in China. The knowledge from deep-water sedimentary research effectively guides the exploration and development of the Chang 7 reservoir formation and remains significant for exploration in the Ordos Basin. The Changqing Oilfield Company made a significant discovery in the Xin’anbian area of the central and western part of the basin in 2014 This represents the first tight field in China with over 100 million tons of proven reserves and highlights the significant exploration potential of the Chang 7 interval [55–57]. The results reveal the characteristics, the sublacustrine fan development, and the significance of the gravity flow sediments for exploration in the Chang 7 interval in the southeastern Ordos Basin.
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