A New Log Evaluation Method to Predict Rich Blocks for Tight Sand Oil
Li, Xia; Zhou, Cancan; Shi, Yujiang; Li, Changxi
Chang-8 oil reservoir in northwestern Ordos basin (Midwest China) is characterized with low to ultra-low permeability smaller than 0.1 mD, which belongs to tight sand oil reservoirs. The reservoir distributes continuously and consists mainly of amalgamated sandstone bodies. However, it has strong lateral heterogeneity and highly variable physical properties and oil saturation. A key method is developed in this study for the optimal selection of oil rich areas and the relationship between hydrocarbon reservoir and source rock through detailed single-well and multi-well interpretation workflow described below.
First, log data standardization and normalization is carried out based on the variation-trend analysis of the marker beds. This process is constrained with core data and geological information. Following that, a fine model of porosity and permeability is established through "core calibration logging", and an oil saturation model is constructed by studying the rock electric property. From these individual well studies, a multi-well log-evaluation is performed and the planar distribution characteristics of porosity, permeability and oil saturation are studied and mapped. Detailed logging interpretation of single well and the reservoir correlation of multi-well along the direction of sand bodies are performed to confirm the distribution characteristics of oil reservoirs.
A characterization parameter (VOIL) which considers the comprehensive effect of physical properties and oil contents is put forward to describe the oil concentration.The Total Organic Carbon (TOC) of source rock in Chang7 formation is evaluated using logs. We use resistivity log to overlap prosity logs such as density log or sonic log in non source rock layers, then the amplituded differnces between resistivity log and prosity logs in source rock layers can be converted to TOC contents using corresponding formula. Some computing parameters in the above method should be calibrated with actual core geochemistry experiment. The collocation relationship between hydrocarbon generating ability of source rocks (characterized by TOC usually) and the oil concentration of reservoir (VOIL) controls the distribution of oil-rich areas. From the superposition diagram of the planner distribution of TOC and VOIL and the relationship between source rock and reservoir, the enrichment regularity of tight sand oil reservoirs is confirmed and the oil-rich blocks are determined.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013