Diagenetic and metamorphic characteristics and implications for hydrocarbon reservoirs in the country rocks influenced by magmatic emplacement： A case study from an outcrop of diabase intrusion in the southern Songliao Basin
Liu Chao1 Xie Qingbin1，2 Wang Guiwen1，2 Song Yifan1 Wang Yuhan1 Tang Yong1
College of Geosciences, China University of Petroleum, Beijing 102249；
State Key Laboratory of Petroleum Resources and Prospecting, Beijing 102249
Magmatism is encountered widely in hydrocarbon-bearing basins worldwide. However, its influences on country rocks as hydrocarbon reservoirs have been controversial. The current study takes an outcrop of diabase intrusion in the Tuanshanzi area, southern Songliao Basin, as a case study, aiming to investigate diagenetic and metamorphic characteristics of country rocks, on the basis of continuous sampling, thin-section observations, scanning electronic images (SEM), electron probe microanalysis (EPMA), fluid-inclusion microthermometry, X-ray diffraction analysis (XRD), and oxygen and carbon stable isotopes. The intrusive outcrop is diabase, and upward country rocks are hornfels and metasandstones. Authigenic sericite is an indicator of metamorphism in hornfels. The sericite content is decreasing with increasing distances from diabase intrusion. Due to heat influence caused by diabase intrusion, the clay matrices are characterized by illite and chlorite, which commonly represent later-stage diagenesis. Vesicle-amygdule structures, which are filled with laumontite, develop exclusively in hornfels. In addition, augite grains are detected in country hornfels. These aspects indicate that the country rocks were weak consolidated when intruding occurred. Diagenetic and metamorphic features of overlying metasandstones influenced by diabase intrusion include: 1） characteristic distribution of authigenic quartz. In locations adjacent to diabase intrusion, authigenic quartz is mainly microcrystalline quartz grains, while in places distal from diabase, authigenic quartz is mainly quartz overgrowth; 2） uneven distribution of clay minerals. The content of mineral contents increases with increasing distances from intrusion body; 3） forming characteristic structure reflecting mechanical pressure, such as cracking and wavy extinction of quartz grains. The mechanical pressure gradually weakens with increasing distances from diabase intrusion. Although minor reservoir pore spaces are developed in hornfels (average porosity of 1.70%), barely no seepage potential in them (average permeability of < 0.1 mD), mainly because that microfissures formed due to hydrothermal fluids breaking are filled completely with calcites. The occurrence of laumontite and depleted kaolinite reflect alkaline diagenetic environment, which prevents the dissolution of calcites in the microfissures. Metamorphism is fundamental for hornfels turning into hydrocarbon reservoirs, while later-stage dissolution is crucial for them to be effective reservoirs. In sandstones, mechanical pressure and hydrothermal fluids flow result in heterogeneous distribution of authigenic minerals, thereby causing or intensifying reservoir heterogeneity.
Liu Chao Xie Qingbin Wang Guiwen Song Yifan Wang Yuhan Tang Yong. Diagenetic and metamorphic characteristics and implications for hydrocarbon reservoirs in the country rocks influenced by magmatic emplacement： A case study from an outcrop of diabase intrusion in the southern Songliao Basin[J]. Chinese Journal of Geology, 2017, 52(2): 453-469.