Sedimentation of the Lower Cretaceous Xiagou formation and Its Response to Regional Tectonics in the Qingxi Sag, Jiuquan Basin, NW China

Under the constraint of an isochronous sequence stratigraphic framework, sediment infill of the Xiagou Formation reflects the overall control of dynamic tectonic movements and episodic sedimentations in the Qingxi Sag. Structure reactivity during post-depositional processes could cause stratigraphic variations in longitudinal time and lateral space. This study documents sediment infill features and their response to the tectonic evolutions of the Qingxi Sag. The data sets include comparison of cores, well drilling, 3D seismic, inter-well correlation, wave impedance inversion profiles, original strata recovery data, sedimentary fades spatial evolution and their superimposition with paleogeomorphology. The Jiuquan Basin is a Mesozoic-Cenozoic superposition basin comprising an early rifting graben phase and a later compression phase. Since the Early Cretaceous, the basin has undergone four major tectonic episodes: 1) extension during the Early Cretaceous, 2) tectonic inversion caused by northwest-southeast contraction from the Late Cretaceous to the Paleocene, 3) weak extension from the Eocene to the Miocene and 4) contraction from the Miocene to the present. Therefore, the Jiuquan Basin is the product of taphrogenic, collisional and shearing movements. Seismic interpretations of sequence and maximum flooding surface divide the Xiagou Formation into three third order sequences: SQK1g(0), SQK1g(1) and SQK1g(2+3). Five sedimentary facies associations are identified: the shoreland plain, fan delta dominated sedimentary systems, turbidite deposits, shallow lakes and half-deep lake systems. From K1g(0) to K1g(2+3), decreased sandstone percentages in three fan delta areas indicate a continuously transgressive process, which shows the transition from proximal to distal sites in most statistic wells and an obvious decrease of fan delta scales. The northeast-southwest faults control the lakeward distributions of delta fronts and turbidite fans. The correspondence of sedimentary infill and its response to tectonic movements have been demonstrated in the Qingxi Sag. The more active eastern part of the northeastern boundary fault has an important influence on the northeastward migration of depocenters in the Xiagou Formation. The topography developed continuously from K1g(0) to K1g(2+3), but the diminished subsidence indicates the dominant geological process varying from intense fault rifting in an early period to relatively gentle and overall subsidence in a later period during the Early Cretaceous. (C) 2013 Elsevier Ltd. All rights reserved.Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education TPR-2011-09Project of "Double strong effect, driving mechanism and hydrocarbon significance of tectonic activity during depositional period of Dongying Formation in Qikou and Nanpu Sag, Eastern China" 41272122Geological Science

Geochemical characteristics of inclusion oils and charge history in the Fushan Sag, Beibuwan Basin, South China Sea

The Fluid Inclusion (FI) petrology and biomarker analysis of the inclusion oils were carried out on three sandstone samples from the Liushagang Formation (Els) reservoirs in the Fushan Sag, the Beibuwan Basin, South China Sea. The purpose of this study is to identify the origin of the inclusion oils and understand the charging history and oil accumulation process in the Els reservoirs through the inclusion oil-source correlation, analysis of Homogenization Temperature (Th) and 1D basin modelling. The results show that the maturities of the Els22 inclusion oils are higher than that of the Els1 inclusion oil based on methylphenanthrene index and the inclusion oils were originated from fluvial/delta mudstone. The geochemical correlation between the inclusion oils and source rocks based on the biomarkers clearly shows that the inclusion oils from the Els22 sandstone reservoirs may have sourced from Els22 source rock or mixed source from the Els3 and Els22 mudstones, and the inclusion oil in the Els1 reservoir was generated from the Els1 source rock in the sag. The oil source of inclusion oils from different layers was almost consistent with that of crude oil in reservoirs. Combined with the Th of the FIs and 1D basin modelling, two episodes of oil charge are identified in the center of the Bailian Sub-sag: the first oil charge in the Els22 reservoirs started around 38.5 Ma in the lower petroleum play, which is consistent with the early oil generation time of the Els3 and Els22 source rocks in the Bailian Sub-sag. The second oil charge in the Els1 reservoir was coincided with the early oil generation time (?13-0 Ma) of Els1 source rock. The oil charge and accumulation occurred in the isolated upper and lower petroleum plays. The oil charge and accumulation in the lower petroleum play from the Els3 and/or Els22 source rocks is almost distributed in the whole sag, and the oil in the upper petroleum play from the Els21 and/or Els1 source rock is mostly distributed in the center of the sag. ? 2023 Elsevier Lt