Outcrop Analog Model of Natural Fractured Carbonate Reservoir: A Case Study from Khuff Formation, Central Saudi Arabia
M.A. Yassin1 and O.M. Abdullatif1
Within tight carbonate reservoir, fractures are very important because they enhance porosity and permeability. The Permo-Triassic carbonates Khuff reservoirs in the Middle East are estimated to contain about 15-20 % of the world's gas reserves. Outcrop analog study is used establish a conceptual model to understand and estimate the fractures in the Khartam member of Khuff Formation. This includes examining and evaluating the fracture types, orientation, length spacing (density), aperture, pattern distribution and bedding thicknesses. The study methodology includes several phases such as geological field survey, terrestrial LiDAR (light detection and ranging) mapping, fractures and facies description and analysis, fracture and matrix porosity (Dual porosity) and permeability, geostatistical analysis, and 3D model construction. Discrete Fracture Network (DFN) models were generated. The observed fractures include tensile and shear fractures more over the pressure solution (stylolites). For better representations of fractured reservoir, model geometry was built based on the top and base horizons interpreted from the LiDAR data. The dimensions of the study area are 1,500 m x 500 m. The cells size is 1 x 1 m. In this study, fractures of the same type and generated at the same time are grouped into a fracture sets and then modeled stochastically using statistical and geostatistical discrete modeling methods based on the fractures properties. The sub-seismic scale fractures were interpreted from LiDAR data, and then modeled deterministically. The majority of fractures orientation is NNW-SSE and WNW-ESE with minor direction NW-SE and NE-SW. The current-day in-situ stress is dominated by the Zagros plate tectonics (E-W). From the in-situ stress analysis, it is believed that the WNW-ESE fractures trends may be the most susceptible to dilation (open) at present day. It is believed that lithofacies variation is controlling the fracture intensity across the Khuff reservoir. Fractures intensities increase in the zones of low porosity and high density. Fractures are more common in mudstones rather than in grainstones and in dolomite rather than grainstones. In the study area, the common lithofacies is mudstones and grainstones with minor dolomite. The fracture outcrop analog model of Khartam member provides data that might help to understand, and predict Khuff fractured reservoirs in the subsurface.
AAPG Search and Discovery Article #90188 ©GEO-2014, 11th Middle East Geosciences Conference and Exhibition, 10-12 March 2014, Manama, Bahrain