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Prediction of Lithologies and Reservoir Quality Using Well Logs in Wolfbone Play in the Delaware Basin, West Texas


The Wolfbone intervals in the Delaware Basin, West Texas were once thought to be the hydrocarbon source rocks. However, large volumes of hydrocarbon have been produced from the Wolfbone play recently due to the successful application of horizontal well drilling and hydraulic fracturing. The complex lithologies (basin floor to slope turbidite sandstones interbedded with hemipelagic mudrocks and finely laminated siltstones) and sedimentary structures increase the heterogeneity of reservoir, leading to the difficulty of detailed reservoir characterization. Porosity prediction using conventional well logs remains an elusive goal as well. This presentation is to characterize the lithologies of the reservoirs based on core and well logs, with emphasis on the lithologies prediction using well logs. Five types of distinctive lithologies are identified by core observation, including (1) fine grained sandstone, (2) laminated siltstone, (3) mudrock, (4) calcareous mudrock and (5) carbonate. Lithologies correlated with the porosity and permeability measured on 100 core plug samples reveal the control of lithologies on reservoir quality. Reservoir properties are poorest in carbonate (por: 2.3%-7.6%; average: 5.6%; perm: 0.001-0.022md; average: 0.013md). The reservoir property of mudrock and calcerous mudrock is better (2.5%-10.3%; 7.8%; <0.001-0.074md; 0.013md). Laminated siltstone show a wide range of porosity and permeability (3.3%-12.6%; 8.9%; 0.00-0.282md; 0.049md). The porosity and permeability of sandstone is highest (6.3%-13.1%; 10.2%; <0.001-0.446md; 0.111md). Also, log values including GR, RHOB, NPHI, DT and RT are obtained in order to calibrate with the corresponding depth of core-measured porosity and permeability. Strong linear relationships are observed between core porosity and RHOB with regression coefficients larger than 80%. Therefore the RHOB can be used to predict the porosity for each lithology in a single well. Thus the ideal interpretation model of porosity by well logs is built based on various lithologies. The lithology distribution of 200 wells in study area is figured out by comprehensive analysis of petrophysical well logs. The porosity for each lithology is calculated by RHOB. At last the 3D porosity and lithologies modeling of reservoir is built. The result sheds light on heterogeneity of the reservoir and provides information for further exploration and exploitation in the Delaware Basin.