Reservoir characterization and geological controls of the Lower Cretaceous sandstones in the northern Wuerxun Sag, Hailar Basin, China

Abstract

The Lower Cretaceous sandstone reservoirs in the northern Wuerxun Sag host abundant hydrocarbon resources and natural CO2 gas. Reservoir quality and heterogeneity are regarded as significant factors affecting the hydrocarbon and CO2 distribution, which have not been comprehensively investigated. In this paper the characteristics of the reservoirs are studied using well log, core, thin section and petrophysical data, and related geological controls are discussed to analyse the factors responsible for the variable reservoir quality. Results show that the Lower Cretaceous sandstones mainly include common sandstones and dawsonite-bearing sandstones, characterized by variable grain size and poor sorting. That is probably caused by the rapidly variable sedimentary facies as a result of short multiple provenances and complex fault system. The reservoir quality is regarded as poor with general low-moderate porosity (ranging from 1.20% to 27.40%, with an average of 12.32%) and low permeability (ranging from 0.01 × 10−3μm2 to 1708.00 × 10−3μm2 with an average of 19.08 × 10−3μm2). The porosity and permeability generally decrease as the burial depth increases. However dawsonite cements have caused local heterogeneity as dawsonite-bearing sandstones are of lower porosity and permeability than comparatively deeper common sandstones. In general, tectonic and depositional evolution provided the heterogeneous lithologic distribution which is the material basis of the reservoir quality evolution. Diagenesis is proposed to be primarily responsible for the reduction of reservoir quality, with compaction being the most important mechanism volumetrically. Cementation also played a role in destroying reservoir quality, with dawsonite cements causing local heterogeneit and closely related to the presence of natural CO2 gas in the Surennuoer strike-slip fault zone. There is a genetic link among the dawsonite cements, inorganic CO2 gas degassed from the magmatic intrusion bodies and the Surennuoer strike-slip fault zone. Generally this study provides a better understanding of the reservoir quality in the northern Wuerxun Sag as well as related geological controls, which can guide subsequent hydrocarbon production activities in the study area.