Rock typing in tight gas sands: a case study in Lance and Mesaverde formations from Jonah field

Abstract

The Jonah field is one of the biggest tight gas sand fields in the Green River basin. Production profiles from its deeper sections show high liquid hydrocarbons close to the Pinedale anticline, especially in Mesaverde and Lance formations. To assess the potential of condensate production, new approaches for rock classi_cation are needed that will allow us to differentiate between discontinuous sandstone layers and the interbedded siltstones. Currently, the only cut off used is for gamma ray: rocks below 75 API are designated as sandstones. Although, significant porosity and permeability variations occur within the sandstone zones, the only criterium used to differentiate between reservoir and non-reservoir rocks is porosity: sandstones with porosity > 6% are considered reservoir quality rocks. Porosity is considered main controlling factor on permeability. A 6% porosity cut off in sandstones was used in net-pay calculations. However, hydraulic rock typing demonstrates permeability is dependent on main pore throat radius, rather than porosity. This study presents rock typing for tight sandstones and siltstones with an understanding of petrophysical properties such as pore structure, porosity, permeability, and cementation. I studied 14 samples from the Mesaverde and Lance Formations with lithologies varying from clean sandstone to mudstone. X-ray diffraction (XRD) mineralogy and mercury injection capillary pressure (MICP) were measured for all samples. NMR transverse relaxation times (T2) at 2 MHz were also measured for 10 water saturated samples. Nitrogen adsorption tests were performed on 8 samples. Ultrasonic velocities from 10 samples were measured at different confining pressure conditions. Thin section petrography was used to analyze the cementation and pseudomatrix clay effects on pore and pore throat size. MICP data are used to subdivide rocks into three groups based on pore throat size distribution: reservoir sandstones, non-reservoir sandstone and siltstone/mudstone. Dominant pore throat size for reservoir and non-reservoir sandstones are 400 and 100 nm, respectively. In order to apply pore throat size rock typing to downhole measurements, NMR pore size classification is used to identify formations. Pore size from NMR demonstrated equivalent behavior to pore throat size from MICP. The logarithmic mean values of T2 transverse relaxation times for reservoir, non-reservoir sandstone and siltstone/mudstone are 22.2 ms, 3.4 ms and 0.29 ms, respectively. Clear separation of reservoir sandstone, non-reservoir sandstone and siltstone is seen based on compressibility behavior from compressional velocity during initial pressure loading. Reservoir sandstone demonstrates the highest compressibility. In addition, siltstone and mudstone were separated based on log differential pore volume distribution from N2 adsorption data. Based on pore size distribution data, four main rock types are identified in Lance and Mesaverde formations in Jonah field. Rock typing based on gamma ray and porosity logs can be considered as rock classification of end members. To capture transitional behavior in between end members, pore size distribution is needed in logging application. Since NMR T2 distribution show similar spectra to MICP throat size distribution, the rock typing technique can be applied using NMR log data. Separation of mudstone from siltstone can be used for identification of shale end points in log data. Porosity and resistivity of shale end points are inputs in water saturation calculations.