Reservoir characteristics and forming controls of intrusive-metamorphic reservoir complex: A case study on the diabase-metamudstone rocks in the Gaoyou sag, eastern China

Abstract

Hydrocarbon reservoirs associated with magmatic intrusion have significance in expanding hydrocarbon exploration. However, this type of reservoir is rarely documented in previous studies. Taking the diabases and their contact metamudstones from the Funing Formation, northern slope of the Gaoyou sag, as study objectives, the current work mainly investigated lithofacies, reservoir porosities and properties, and C, O isotopes of intrusive diabases and/or contact metamudstones, aiming to unravel reservoir characteristics and forming controls of diabase and metamudstones. After diabase intrusion, four lithofacies, i.e., diabase center, diabase margins, slates, and hornfels, were formed. Meanwhile, various porosity types were developed, including primary vesicles, grain-dissolved micropores, contractive micropores, cooling fractures, and structural fractures in the diabases, and recrystallizational and matrix-dissolved micropores, hydrothermal and contractive microfractures, and structural fractures in the contact metamudstones. Controls on diabase reservoir development include uneven cooling, dissolution, authigenic minerals filling, and structural movements. For contact metamudstone reservoirs, the forming controls include physical fracturing, hydrothermal baking, and authigenic carbonate dissolution. The controlling factors exerted diverse influences on reservoir properties with respect to different parts in contact metamudstones or diabase, thus causing obvious heterogeneity regarding to reservoir quality, i.e., hornfelses and diabase margins exhibiting preferable reservoir properties than slates and diabase center, respectively. Finally, an evolutional model depicting the forming process of diabase-metamudstone reservoir complex was tentatively proposed.