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To understand the evolution of the Miocene gravity flow deposits in the Lower Cong—Congo Fan Basin, this paper documents the Miocene sequence stratigraphic framework, the depositional characteristics and the controlling factors of the gravity flow system. Based on the establishment of high-resolution sequence stratigraphic framework, lithofacies characteristics and sedimentary units of the gravity flow deposits in the region are identified by using seismic, well logging and core data comprehensively, and the sedimentary evolution process is revealed and the controlling factors are discussed. The Miocene can be divided into four 3rd-order sequences (SQ1—SQ4). The gravity flow deposits mainly include siliciclastic rock and pelite. The main sedimentary units include slumping deposits, mass transport deposits (MTD), channel fills, levee-overbank deposits, and frontal lobes. In the Early Miocene (SQ1), mainly gull-wing, weakly restricted to unrestricted depositional channel-overbank complexes and lobes were formed. In the early Middle Miocene (SQ2), W-shaped and weakly restricted erosional-depositional channels (multi-phase superposition) were subsequently developed. In the late Middle Miocene (SQ3), primarily U-shaped and restricted erosional channels were developed. In the Late Miocene (SQ4), largely V-shaped and deeply erosional isolated channels were formed in the study area. Climate cooling and continuous fall of the sea level made the study area change from toe of slope-submarine plain to lower continental slope, middle continental slope and finally to upper continental slope, which in turn affected the strength of the gravity flow. The three times of tectonic uplifting and climate cooling in the West African coast provided abundant sediment supply for the development of gravity flow deposits. Multistage activities of salt structures played important roles in redirecting, restricting, blocking and destroying the gravity flow deposits. Clarifying the characteristics, evolution and controlling factors of the Miocene gravity flow deposits in the Lower Congo—Congo Fan Basin can provide reference for deep-water petroleum exploration in this basin.

Based on the Late Paleozoic geological background and the latest exploration achievements of the Ordos Basin and North China platform, it is concluded that during the sedimentary period of Permian He 8 Member, the area in concern had multiple material sources, multiple river systems, flat terrain, shallow sedimentary water, widely distributed fluvial facies sand body and no continuous lake area, so alluvial river sedimentary system developed in the whole region. Based on stratigraphic correlation and division, and a large number of drilling and outcrop data, a comprehensive analysis of lithofacies and sedimentary facies types and distribution was carried out to reconstruct the ancient geographic pattern of the He 8 Member sedimentary period. The results of paleogeography restoration show that the area of Ordos Basin was the “runoff area” in the sedimentary slope in the western part of the North China platform during the sedimentary period of He 8 Member, the whole region was mainly alluvial plain sedimentation featuring alternate fluvial facies, flood plain facies and flood-plain lake facies. According to the results of flume deposition simulation experiment, a new sedimentary model of “alluvial river & flood-plain lake” is established, which reveals the genesis of large area gravel sand body in He 8 Member of this area and provides geological basis for the exploration of tight gas in the south of the basin. Key Words: Ordos Basin, He 8 Member, North China platform, paleogeographic restoration, alluvial river, flood-plain lake, sedimentary model, exploration significance

Abstract: The geologic structure of deep formations, basic geologic pattern of fault depressions, geologic conditions of Lower Cretaceous Shahezi Formation tight gas reservoir, and the exploration direction in the next step in the Songliao Basin are examined based on a comprehensive and comparative analysis of the outcrop, drilling, seismic and geochemical data. Systematic comparison analysis shows that the geologic structure of deep formations in the basin is characterized by a double-layer structure with fault depression in the lower layer and depression in the upper layer. In the deep fault depression (DFD) stage, fault depressions bounded by faults or onlaps and uplifts, developed independently. The faults related to the formation and evolution of the fault depressions can be divided into regional discordogenic fault, basement depression controlling fault, and secondary fault inside depression, and their actions in the evolution of the fault depression groups are presented in the paper. Based on analysis of regional tectonics and basin evolution features, three exploration fields, tight conglomerate gas, volcanic rocks and basement buried hills are pointed out, and it is concluded the tight conglomerate gas in the Shahezi Formation is currently a new strategic field with the most promising exploration prospect. Based on comprehensive analysis of basic oil geologic features of Shahezi Formation, it is proposed that conglomerate is the main category of clastic reservoir in Shahezi Fromation, and parent rock composition, sedimentary structure and facies, and diagenesis are the main factors affecting the distribution of conglomerate reservoirs. Furthermore, the gas reservoirs in Shahezi Formation, all source and reservoir in one, include two types, lithologic and structural types. Favorable sedimentary facies belts in the gentle slope and steep slope control the distribution of the reservoirs, formation of lithologic traps and gas reservoir scale; while secondary faults inside fault depressions control the formation of structural traps and enrichment of natural gas. Finally, seven fault depressions in Shahezi Formation with the most promising prospect of tight gas have been sorted out through comprehensive evaluation. Key words: Songliao Basin, deep geologic structure, Cretaceous, Shahezi Formation, tight conglomerate gas, exploration prospect