Characterization of organic matter fractions in an unconventional tight gas siltstone reservoir

Abstract

This paper on core samples collected from the Triassic Montney Formation tight gas reservoir in the Western Canadian Sedimentary Basin (WCSB) illustrates that operationally-defined S1 and S2 hydrocarbon peaks from conventional Rock–Eval analysis may not adequately characterize the organic constituents of unconventional reservoir rocks. Modification of the thermal recipe for Rock–Eval analysis in conjunction with manual peak integration provides important information with significance for the evaluation of reservoir quality. An adapted method of the analysis, herein called the extended slow heating (ESH) cycle, was developed in which the heating rate was slowed to 10°C per minute over an extended temperature range (from 150 to 650°C). For Montney core samples within the wet gas window, this method provided quantitative distinctions between major organic matter (OM) components of the rock. We show that the traditional S1 and S2 peaks can now be quantitatively divided into three components: (S1ESH) free light oil (S2aESH) fluid-like hydrocarbon residue (FHR), and (S2bESH+residual carbon) solid bitumen (more refractory, consolidated bitumen/pyrobitumen).The majority of the total organic carbon (TOC) in the studied Montney core samples consists of solid bitumen that represents a former liquid oil phase which migrated into the larger paleo-intergranular pore spaces. Physicochemical changes to the oil led to the precipitation of asphaltene aggregates. Subsequent diagenetic and thermal cracking processes further consolidated these asphaltene aggregates into “lumps” of solid bitumen (or pyrobitumen at higher thermal maturity). Solid bitumen obstructs porosity and hinders fluid flow, and thus shows strong negative correlations with reservoir qualities such as porosity and pore throat size.Although the FHR fraction constitutes a small portion of the total rock mass and volume in Montney samples it has important implications for reservoir quality. This fraction represents a thin film of condensed, heavy molecular hydrocarbon residue covering surfaces of the present-time pore spaces and may represent the lighter component of the paleo-oil that migrated into tight interstices in the Montney reservoir. The FHR fraction potentially plays an important role in wettability alteration by creating hydrophobic matrix pore networks in portions of the reservoir that were not already filled with solid bitumen.