Abstract

A comprehensive sedimentary and reservoir analysis was conducted based on seismic, well logging, core and relative test data, taking Members 1 and 2 of Shahejie Formation of the early Oligocene in the steep slope belt, eastern Shijiutuo Uplift (STU), Bohai Bay Basin (BBB) as a case. The study indicates that a near-shore mixed fan deposit formed in the study area and developed characteristics and pattern of a high-quality reservoir. The mixed clastic-carbonate rocks constitute Members 1 and 2 of Shahejie Formation which developed along the steep slope belt and is named as a near-shore mixed fan. The mixed fan of the study area is mainly composed of microfacies of proximal channel, mixed deposited channel, mixed clastic beach, mixed bioclastic (grain) beach, with vertical multi-stage superimposition feature, and basically a similar shape as modern near-shore fans. It constitutes a new depositional type developing in the steep slope belt of a characteristic and complex lacustrine rift basin in the study area. This mixed fan in the steep slope of eastern STU is controlled by comprehensive factors including tectonics, clastic material supply, climate, palaeogeomorphology and hydrodynamic conditions. The reservoir quality of Members 1 and 2 of Shahejie Formation of eastern STU is, however, actually controlled by the sedimentary environment and diagenesis processes. Coarse-grained mixed rocks of near-shore fans, rich in bioclastics, can form excellent reservoirs, characterized by resistance to compaction, easy to dissolution, little influenced by burial depth and high production of oil and gas, which enable them become key exploration targets of medium-deep strata of BBB. Analyses of high-quality reservoir, its controlling factors and the oil and gas exploration implications of the near-shore mixed fan developing in the study area give a deeper insight into discussions of the same type of mixed rocks of other lacustrine rift basins worldwide.

Highlights

  • Terrigenous clastics and carbonate rocks are separated as two systems in traditional sedimentary researches, which consider that infilling of terrestrial materials will inhibit deposition of carbonate rocks and clastic rocks and carbonate rocks deposit in an opposite tendency

  • In a narrow sense, mixed deposition refers to the compositional mixing of terrestrial clastics and carbonate rocks, i.e., the mixture of component and texture forming the typical product of mixed sediments

  • 7 Conclusions Near-shore mixed fans developed in Members 1 and 2 of Shahejie Formation of eastern part of Shijiutuo Uplift (STU), Bohai Bay Basin (BBB), which are dominated by thick sandy conglomerate lithofacies rich in bioclastics

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Summary

Introduction

Terrigenous clastics and carbonate rocks are separated as two systems in traditional sedimentary researches, which consider that infilling of terrestrial materials will inhibit deposition of carbonate rocks and clastic rocks and carbonate rocks deposit in an opposite tendency. In most cases, these two types of sediments will not mix with each other which has led to insufficient attention being paid to mixed sediments ( known as mixosedimentite in China). It refers to the interbedding and interlayering in a strata profile due to the frequently alternating of terrestrial clastics and carbonate rock deposits, i.e., mixed deposited series. In a narrow sense, mixed deposition refers to the compositional mixing of terrestrial clastics and carbonate rocks, i.e., the mixture of component and texture forming the typical product of mixed sediments

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