Characteristics and formation of high quality reservoirs in sediment gravity flows of Gangzhong area, Huanghua depression

Abstract

Reservoirs can be developed in the sediment gravity flows. However, high quality reservoirs are found widespread in sediment gravity flows of Gangzhong area, Huanghua depression, Bohai Bay Basin, East China. Characteristics and formation of these reservoirs are key problems to be solved. Through comprehensive analysis of thin section petrography, scanning electron microscopy and X-ray diffraction, two distinct rules were obtained. 1) These high quality reservoirs have apparent characteristics: lithology consists mainly of medium-fine grained sands; moderately-well sorted and rounded; intergranular pores dominating >70% of the entire pores, surface per unit pore volume reaches 15%; average porosity is 21% and average permeability is 55×10⁻³ μm². 2) Types of sedimentary microfacies and dissolution strongly control on the formation of high quality reservoirs. Main channels and sandy braided bars have the best reservoir properties. Because that sediments are mainly medium-fine grained sands in high-energy environments. The favorable primary porosity and permeability may promote calcite cementation and help to produce more secondary pores. Besides, at the depth of 2500–3200 m, basically matching threshold of oil generation, organic acid expelled when organic matter became mature, and H⁺ released during clay mineral transformation. These both result in the dissolution of calcite cements and create large volume pores, then physical properties improve correspondingly. Moreover, deep hydrothermal fluid intrusion may also have impacts on the development of secondary pores.

Keywords

Huanghua depression / sediment gravity flows / diagenesis / intergranular pores / hydrothermal fluid intrusion

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