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Paper Number: 106

Gravity current deposits: dominant reservoirs of shale oil in post-rift basins - insight from the Late Cretaceous Qingshankou and Nenjiang Formations, Songliang Basin, Northeastern China

Youliang Feng¹, Hongjun Wang¹, Shun Zhuang², Guibao Liu¹, Xiuli Fu²

1. Research Institute of Petroleum Exploration and Development, Petrochina, Beijing, 100083, China

2. Research Institute of Petroleum Exploration and Development, Corporation of Daqing Oilfield, Petrochina, Daqing, Heilongjiang Provence, China

Songliao Basin is a large Mesozoic nonmarine petroliferous rift basin in northeastern China. The thickest deep lake source rocks were developed in Late Cretaceous post-rift strata of the Basin. Based on observation of cores and interpretations of high resolution 3D seismic data, deep water gravity current deposits (GCD) are identified within the first member (K₂qn¹) of Qingshankou Formation (Turonian) and first to second members (K₂n^1-2) of Nenjiang Formation (Campanian).

During Sq_qn¹ ₍K₂qn¹), northern Xiaoxinganling and western Daxinganling mountains were predominant provenances of the northern and western deltaic systems, respectively. The gravity current channels, fed by northern deltaic systems within TST_qn1 to early HST_qn1 and composed of trunk channels and tributary, extended a very long distance along faults from north to south. These channels terminated in bifurcations or small fans. There are slope fans  composed of slump deposits, sandy debris flow, muddy debris flow and low-density flow, fed by the western deltaic systems,  and controlled by flexural slope-break zone at western slope of the Qijia-Gulong depression within Sq_qn1 (K₂qn¹) too.

During Sq_n1-2 (K₂n^1-2), because of uplift of Zhuangguangcailing Mountain influenced by far-field effect caused by the oblique subduction of the Izanagi Plate under the Eurasian plate, the northern Xiaoxinganling, eastern Zhuangguangcailing mountains and southeastern uplift became dominant provenances. Channel-levee systems, fed by northern deltaic subaqueous distributary channels, extended long distance from north to south in TST_n1-2. Oblique progradational reflection configuration of eastern deltaic systems from east to west within HST_n1-2 on seismic profiles indicates that lake level fell quickly with falling trajectory of the lake shoreline. Meanwhile, large scale mass transport deposits (MTD) were deposited on maximum flooding surface of the sequence.

Three factors influenced on the GCD: 1) lake level rise, 2) flexural slope break/palaeoreliefs, and 3) falling trajectory of the lake shoreline. The sand bodies of the GCD, which are encased within organic-rich source rocks, provide new targets of shale oil for hydrocarbon exploration and development.