The common belief among many petroleum geologists that regions of volcanic and metamorphic rocks are generally to be avoided as potential hydrocarbon reservoirs has greatly slowed the research and exploration efforts on hydrocarbon potential in volcanic and metamorphic rocks. However, many hydrocarbon-bearing basins containing volcanic and metamorphic rocks have been found in convergent margin settings and in rift basins. This article describes the reservoir lithofacies and wire-line logs and elucidates the parameters controlling reservoir-quality evolution of Archean metamorphic and Jurassic volcanic rocks from the Xinglongtai buried hill, western depression of the Liaohe basin, China. Four lithofacies (pyroclastics, lavas, volcaniclastics, and volcaniclastic-epiclastics) have been identified in the Jurassic volcanic reservoir rocks, each having different pore types and variable porosity and permeability values and, thus, different reservoir potentials. Pore types in the volcanic rocks include voids, fractures, fissures, weathering cracks, interstices, and vesicles. The volcanic-rock reservoir evolution is primarily controlled by the burial-thermal diagenesis. Plastic deformation and alteration of the biotite during the eogenetic phase led to the considerable loss of primary pores. Destruction of the primary porosity by compaction was limited by the presence of eogenetic carbonate and zeolite cement formation. Dissolution during the deep-burial mesogenetic phase and during near-surface leaching and erosion in the intervening volcanic eruptions enhanced the permeability and increased reservoir quality.
The pore types in the Archean metamorphic reservoir include fractures, dissolution voids, and weathering fissures. Where the Jurassic volcanic rocks or the Paleogene source rocks directly cover the weathered zone, the fissures and fractures have remained open, but where the metamorphic rocks are covered by the Mesozoic mudstones, most fissures are filled with mud and iron oxides. Reservoir quality of the Archean metamorphic and Jurassic volcanic rocks is also partly related to the paleogeomorphology of the area. Rocks in the paleohighs and in adjacent transitional areas have enhanced reservoir properties greater than those in paleolows because of more extensive weathering and the development of vugs and fissures.