Sequence stratigraphy,sedimentary facies and reservoir quality of Es4s,southern slope of Dongying Depression,Bohai Bay Basin,East China

Thin-bedded beach-bar sandstone reservoirs are common in the Eocene Shahejie Formation (Es4s) of Niuzhuang Sag, along the southern gentle slope of the Dongying Depression. Here we report on the link between sequence stratigraphy, sedimentary facies and diagenetic effects on reservoir quality. Seismic data, wireline logs, core observations and analyses are used to interpret depositional settings and sequence stratigraphic framework. Petrographic study based on microscopic observation of optical, cathodoluminescence (CL), confocal laser scanning (LSCM) and scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses were used to describe the fabric, texture, allogenic and authigenic mineralogy of these highly heterogeneous sandstone reservoirs. The Es4s interval is interpreted as third-order sequence, composed of a lowstand systems tract (LST), a transgressive systems tract (TST) and a highstand systems tract (HST). On the fourth order, twenty-nine parasequences and seven parasequence sets have been identified. Sand bodies were deposited mainly in the shoreface shallow lake beach-bar (clastic beach-bar), semi-deep lake (carbonate beach-bar) and the fluvial channels. The depositional and diagenetic heterogeneities were mainly due to the following factors: (1) fine grain size, poor sorting, and continuous thin inter-bedded mud layers with siltstone/fine-sandstone having argillaceous layers in regular intervals, (2) immature sediment composition, and (3) even with the dissolution of grains and several fractures, destruction of porosity by cementation and compaction. Secondary pores from feldspar dissolution are better developed in sandstones with increased cementation. Grain coating smectite clays preserved the primary porosity at places while dominating pore filling authigenic illite and illite/smectite clays reduced permeability with little impact on porosity. Due to the high degree of heterogeneity in the Es4s beach-bar interval, it is recognized as middle to low permeable reservoir. The aforementioned study reflects significant insight into the understanding of the properties of the beach-bar sands and valuable for the comprehensive reservoir characterization and overall reservoir bed quality.     

Structural patterns of the late Mesozoic crustal detachment system in the Raoyang Sag,Bohai Bay Basin,eastern China: New insights from 3D seismic data

Understanding the late Mesozoic tectonic origin and structural patterns of the Bohai Bay Basin (BBB) is of growing importance for its buried-hill petroleum exploration. This study revealed, based on 3D seismic and borehole data, that the Raoyang Sag in the Jizhong Depression, western BBB, was dominated by a crustal detachment system during the late Jurassic to the early Cretaceous. It was characterized by the WNW-dipping low-angle detachment fault F₀ (namely Cangxi Fault), the structural dome cored by Archean Basement rocks at the footwall and supradetachment basins on the hanging wall. We suggested the late Mesozoic volcanic materials and coarse deposits accumulated in the supradetachment basins had locally diminished the petroleum prospect, but the syndetachment deformation and denudation had contributed to it by means of promoting petroleum migration from source rocks to reservoirs and improving porosity and permeability of the reservoir rocks by karstification and structural fracturing.