No abstract

The beach‐bar reservoir play has become an important exploration target within the Bohai Bay Basin, especially in the Boxing Sag within the Dongying Depression, where a large‐scale lacustrine beach‐bar oil pool has been discovered recently. The sedimentary characteristics, distribution patterns and formation mechanisms of beach‐bar sand bodies in the upper fourth member of the Eocene Shahejie Formation (Es4s) in the Boxing Sag were studied in detail based on seismic, well log data and core data. The Es4s in the Boxing Sag is composed of a third‐order sequence consisting of three systems tracts, i.e. a lowstand systems tract, a transgressive systems tract and a highstand systems tract. Beach‐bar sand bodies were deposited widely in the basin during the lowstand systems tract period. The sandy beach‐bars are characterized by siltstones, fine‐grained silty sandstones interbedded with thin mudstone units. The presence of well‐developed sedimentary structures, such as swash bedding, parting lineation, parallel bedding, ellipsoidal mud clasts, ripples, terrestrial plant debris and vertical burrows, suggests that beach‐bars were deposited in a relatively shallow water environment under the influence of strong hydrodynamics. Laterally, the sandy beach facies occurred as a more continuous sheet‐like body around the sandy bar in most parts of the sag. Stratigraphically, beach‐bars were distributed mainly in the lowstand systems tract and they were less well‐developed in the transgressive systems tract and highstand systems tract. Several factors were probably responsible for the occurrence of the large‐scale beach‐bars during the lowstand systems tract period, including: (i) a gentle palaeoslope and relatively weak structural activities; (ii) a shallow‐water condition with a strong hydrodynamic environment; (iii) high‐frequency oscillations of the lake level; and (iv) an abundant terrigenous clastic feeding system with multiple‐point and linear sediment sources.

No abstract

No abstract

The genetic type and hydrocarbon accumulation conditions of glutenite in Shengtuo Area have been studied based on cores, well-logs, and formation test data. During deposition of the fourth member of Eogene Shahejie Formation (Es4), Shengtuo Area was predominated by nearshore subaqueous fan and sublacustrine fan, controlled by the Chennan Fault and the Tuo-Sheng-Yong Fault. Nearshore subaqueous fan and sublacustrine fan were respectively developed on the downthrown of the Chennan Fault and the Tuo-Sheng-Yong Fault. Both of nearshore subaqueous fan and sublacustrine fan can be subdivided into three subfacies, including inner fan, middle fan and outer fan, from proximal to distal provenance. The braided channel within the middle fan, characterized by conglomeratic sandstones and sandstones with high secondary porosity and overpressured microfissures resulted from the effect of organic acid and reservoir overpressure, is the best reservoir among these three subfacies. Inner fan subfacies, consisted of thick-bedded, poorly-sorted and matrix-supported conglomerates, has low primary and secondary porosity, is the best sealing among this depositional system. Outer fan sediments, comprised of thick dark mudstones interbedded with thin sandstones in which overpressured microfissures developed very well, accounts for the migration pathway for oil and gas to enter braided channel reservoir from source rocks. In addition, the glutenite fans were developed near the Lijin Sag and the Minfeng Sag, which deposited two sets of high quality source rocks (Es4s (upper section of Es4) and Es4x (lower section of Es4) source rock). With regional cap rock and appropriate preservation conditions, the different subfacies in glutentite fans worked together and formed a "self-generation and self-storage" lithostratigraphic trap characterized by "migrating through outer fan, accumulating in middle fan and sealing by inner fan".