Geophysical characteristics of internal weathered volcanic crust in the Songliao Basin

Abstract

Weathered volcanic rocks have higher porosity, and weathered volcanic rocks are more likely to form favorable reservoirs and high-yield oil and gas reservoirs. The search for volcanic weathering crust reservoir is of great significance for oil and gas exploration. This paper presents an analysis of Yingcheng formation volcanic features in the Xujiaweizi Fault Depression of the Songliao Basin based upon well logging, drilling, 3D seismic data and analysis of the tectonic setting. This data comes from Jilin Oilfield Company. Because of the large buried depth of volcanic rocks, logging properties and seismic properties are of great significance to the identification of volcanic weathering crust. The geophysical characteristics of the internal weathered volcanic crust in the Songliao Basin, e.g., characteristics of logging curves, imaging logging, seismic profiles, and seismic attributes, are summarized. The data suggest that the logging is characterized by a “three high, two low” pattern, with a large borehole diameter, high natural gamma, high acoustic slowness, low density, and low resistivity. It is an important symbol for finding the weathered crust of underground volcanic rocks. The borehole diameter is 9–16 cm, the natural gamma reading is 140–260 API, the acoustic slowness is 60–120 us/m, and the density is 1.2–2.7 g/cm³. The seismic signature reflects a strong, continuous-low frequency event, which is truncated in deeper formations at the basin boundary. The degradation strata are identified at the margin of Songliao Basin, and the lower strata are continuous. We predicted the total thickness of weathered volcanic crust by using borehole data and geophysical characteristics. We found that the largest thickness was 110 m and that the weathered volcanic crust was clustered in the middle of the Xuzhong zone. Chemical weathering can increase the porosity of volcanic rocks. It has a better effect on volcanic reservoir. We predict that the existence of the weathering crust has a significant relationship with oil and gas reservoirs. These findings may similarly be applied to other weathered volcanic crusts and the characteristics and distribution of volcanic weathering crust have important theoretical and practical significance for the study of volcanic oil and gas exploration.