Origin and reservoirs characterization of lacustrine carbonate in the Eocene Dongying Depression, Bohai Bay Basin, East China. (28th July 2016)

• Record Type: Journal Article
• Authors: Yang, Yongqiang; Qiu, Longwei; Yu, Kuanhong
• Is Part Of: Geological journal. Volume 52:Number 5(2017)
• Page Start: 808
• Page End: 827
• Publication Date: 2016-07-28
• Subjects: lacustrine carbonates -- reservoir characteristic -- diagenesis -- Dongying Depression -- Eocene; Geology -- Periodicals; 551
• DOI: 10.1002/gj.2848
• Languages: English
• ISSNs: 0072-1050

Abstract

Abstract : With the recent discovery of new oil fields around the Dongying Depression, Bohai Bay Basin, a better understanding of the origin of its high‐quality lacustrine carbonate reservoirs has become important. The lacustrine carbonates of the upper fourth member of the Eocene Shahejie Formation (ES4s) have been examined using a variety of methods, including core and thin section observations, XRD, SEM, CL and fluid inclusion and isotope testing. The δ 13 C and δ 18 O values of the Dongying carbonates differ significantly from Eocene marine values, and the 87 Sr/ 86 Sr ratios of carbonate minerals are higher than Eocene marine water and the same as the surrounding Palaeozoic carbonate bedrock. Additionally, the distribution of the carbonates is controlled by the surrounding Palaeozoic carbonate bedrock, all of which indicate their lacustrine origin. Six major carbonate lithofacies are distinguished in the southern gentle slope zone. Bioclastic, oolitic and intraclastic grainstones dominate the littoral to sublittoral zones, while mudstones are mainly distributed in the profundal zone. The ES4s interval is composed of a third‐order depositional sequence that contains several small‐scale, higher frequency depositional cycles. Overall, stacking patterns of high‐frequency depositional cycles are consistent with the general facies variation from basin to shore. Shallow‐water grainstones contain both primary intergranular and secondary dissolution pores, whereas the dolomitized mudstones in the profundal zone contain intercrystalline pores and fractures. Diagenetic processes that improve reservoir quality include dissolution and dolomitization. Reservoir quality was diminished by burial calcite cementation and compaction. High‐quality reservoirs are mostly distributed in dolomitized grainstones and wackestones, which are deposited in the shoal facies of shallow lakes. Variations in reservoir quality in the Dongying Depression are due to a combination of lithofacies type, dissolution intensity, early dolomitization and tectonic‐related deformation. Due to the influx of meteoric waters and shallow burial depth, the grainstones in the littoral zone have the best reservoir quality. Integrated studies have helped to understand the reservoir heterogeneity and hydrocarbon potential of the Dongying lacustrine carbonates. Copyright © 2016 John Wiley & Sons, Ltd.