Mesoscopic Characterization of the Heterogeneity within Tight Carbonate Gas Reservoir, Outcrop Study, Saudi Arabia

The tight carbonate Units within the Permo-Triassic Khuff Formation in the Middle East are considered the most prolific unconventional gas reservoir in the world. The Khuff reservoirs in Saudi Arabia are known for their complex vertical and lateral heterogeneity at the mesoscopic scale that is beyond the capability of the seismic data. Equivalent strata are well exposed in central Saudi Arabia with a perfect lateral continuity of layers. The main objective of this work is to characterize the heterogeneity at the mesoscopic scale from these outcrops to provide insights and reflections about the intra-reservoir heterogeneity in the subsurface. Three reservoir analog Units were selected based on their lithofacies and depositional setting and been sampled laterally with 5 m spacing. These Units were given the names A, B and C from the bottom to the top. The Units are deposited above each other, A is at the bottom of the outcrop and C is at the top of it. 1.5-inch core plugs were prepared from all the collected 90 samples for petrophysical measurements (porosity and permeability). The porosity and permeability results were analyzed using basic statistics and geostatistical measurements including: Coefficient of Variation (Cv), Dykstra-Parsons Coefficient (VDP) and Lorenz Coefficient (Lc). Also, rock-typing methods including Winland (R35) Method, Flow Zone Indicator (FZI) and Global Hydraulic Element (GHE) were utilized to classify the studied geobodies into flow zones (rock types). All the results of the basic statistics, geostatistical measurements and rock-typing methods were integrated to provide a better understanding of the heterogeneity within these Khuff reservoir analogs. The porosity values within the studied Units have a homogeneous to low heterogeneous distribution with C being the most homogeneous and A and B the least heterogeneous. Permeability showed a heterogeneous to very heterogeneous distribution, where A being low heterogeneous while B and C are very heterogeneous. Based on R35 values, these Units were subdivided into three flow Units (or rock types). The FZI combined with GHE method resulted in classifying A into 4 GHE from 3-6, while B has been classified into 6 GHE Units from 1-6. Five GHE Units from 1-5 have been identified in Unit C. The number of rock types classified are related to their lithofacies and depositional setting. The more the number of the rock types identified within the Unit, the more the degree of heterogeneity. From these results, we inferred that B of the intertidal channels is the most heterogeneous and A of the intertidal/subtidal setting is the least heterogeneous and C of the shoal ridges is moderately heterogeneous. The results of this outcrop analog study, if integrated with subsurface data, will provide a better understanding and prediction of the reservoir heterogeneity and rock typing at high-resolution scale within the inter-well spacing as controlled by lithofacies, architecture and depositional setting.