A Novel Method To Identify Surface Water Invasion Hydrocarbon-Bearing Potential Reservoirs Using Geophysical Well Logging Data

Surface water invasion reservoir was defined as such type of reservoir that connected to surface due to tectonism (e.g., denude, discordance) or fractures. Surface fresh water with ultra-low concentration was invaded into formation pore spaces, and this led to the coexistence of high resistivity water-saturated layers and low-resistivity hydrocarbon-bearing reservoirs. It was a great challenge to identify such type of hydrocarbon-bearing potential reservoirs by using geophysical well logging data based on common methods. A large number of surface water invasion hydrocarbon-bearing reservoirs was developed from Yan 6 to 9 Formation in Pengyang Region, southwestern Ordos Basin due to the complicated tectonism and diagenesis in Late Jurassic. The surface fresh water entered formation pore space along the fractures, this led that many fresh water-saturated layers were mis-identified as hydrocarbon-bearing reservoirs due to the high resistivity log response. However, the true hydrocarbon-bearing formations, located in normal formations that isolated from the surface due to hardly any fractures, were discarded. In this study, combining with geologic structural features, drill stem test (DST) data, formation water analysis and conventional well logging data, the distribution area of surface water invasion reservoirs were captured. The difference of the spontaneous potential (SP) and natural gamma ray (GR) log response between such type of formation with common reservoir was extracted, and an assembly parameter based on SP and GR data was proposed to effectively identify surface water invasion reservoirs from the normal formation. Based on the DST and geophysical well logging data, the normalized apparent limestone porosity difference of neutron and density, and normalized apparent limestone porosity difference of acoustic and density was found to be sensitive to indicate hydrocarbon-bearing reservoirs. The crossplot of normalized apparent limestone porosity difference of neutron and density versus normalized apparent limestone porosity difference of acoustic and density was established to identify hydrocarbon-bearing formation. Field applications illustrated that comparing with common methods, the accuracy of identifying such type of hydrocarbon-bearing formation was much improved from 54.2to 91.3%. This was of great importance in indicating the hydrocarbon-bearing reservoirs exploration and development in our target region.