Reservoir properties and controlling factors of contact metamorphic zones of the diabase in the northern slope of the Gaoyou Sag, Subei Basin, eastern China

Abstract

Listen

Translate article

Hydrocarbons reserved in the contact metamorphic zones of magmatic rocks are found in rift basins as a new type of unconventional resources. However, their reservoir characteristics are relatively underexplored. On the basis of core samples, thin sections, and analysis data available, this study investigates the reservoir properties of contact metamorphic zones of the diabase from the northern slope of the Gaoyou Sag, Subei Basin in eastern China. The factors that control the reservoir quality are also investigated. In the studied area, diabase intrusion exerts significant influences on contact sedimentary rocks (including contact mudstones and sandstones) as hydrocarbon reservoirs. After contact metamorphism, the impermeable mudstones are altered into permeable metamudstones, including intermediate-metamorphosed hornfelses and low degree-metamorphosed slates. Various types of reservoir spaces, including structural fractures, hydrothermal microfissures, contraction microfissures, cleavage microfissures, intercrystalline micropores, and dissolved micropores, are created in metamudstones. Among them, numerous fractures and microfissures are the main factors of reservoir quality enhancement. Contact sandstones are altered into metasandstones, whose reservoir space types are similar to those of unaffected sandstones. The reservoir spaces mainly include primary intergranular pores, dissolved intergranular pores, dissolved intragranular pores, moldic pores, ultralarge pores, and dissolved micropores.The formation of reservoir spaces in contact metamudstones is mainly controlled by diabase intrusion, and it is not influenced by burial compaction. The controlling mechanisms of this formation include physical fracturing, hydrothermal activity, dissolution, and cooling contraction. The reservoir quality of contact metasandstones is primarily controlled by lithofacies and diagenesis before intrusion. However, later-stage diabase intrusion also exerts considerable control on reservoir quality by causing physical pressure, dissolution, and cementation. Although dissolution increases porosity moderately, it insufficiently compensates for the porosity reduction caused by physical pressure and cementation. Thus, the reservoir quality of contact metasandstones is generally decreased by diabase intrusion. A comprehensive reservoir model of contact metamorphic rocks is proposed based on the preceding analysis. This model serves to predict the distribution of high-quality reservoir in contact metamorphic rocks in the studied area. It can also be applicable to other formations of similar intrusive background.