Levine, Jeffrey R.¹ 
(1) Consultant Geologist, Richardson, TX

ABSTRACT: Methodology for Integrated Calculation of Total Gas-in-Place for a Heterogeneous OM-Bearing Sequence

Unconventional gas reservoirs are usually classified according to the predominant source of the gas—coal, shale, or tight sand—but recent experience has shown that most are actually "hybrid" systems, producing gas from more than one rock type. A thorough geological evaluation should incorporate the gas resource potential of the entire system, including both “free gas” (residing in conventional porosity) and “sorbed gas”, (dissolved or adsorbed on sedimentary organic matter; OM)). OM may occur either in concentrated layers (coal) or dispersed in shales & mudstones. Sorbed gas capacity is proportional to OM content, OM type, thermal maturity, pressure, temperature, and gas saturation, all of which vary both vertically and laterally. Methodologies currently in use are inadequate to take these variations into account. 
The procedure described herein utilizes a combination of core data, lab data, and geophysical log data to estimate the OM content, thickness, density, and sorbed gas content of all potential gas-bearing units. Sampling and core analysis programs must be properly designed to provide the required data. Four different categories of OM-bearing rocks are identified automatically by log analysis. Results compare favorably with hand “picks” made by experienced geologists. 
Gas content is determined layer-by-layer, then integrated to yield a total resource estimate. Cumulative gas resource plotted versus depth can be used in economic modeling or in drilling and completion design, assessing the gas resource potential of new prospects, planning well spacings or completions, refining reserve estimates, and recognizing the potential for infill drilling or recompletions.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.