--> Organic Matter Macerals and Associated Mineralogical Matrices—Marcellus Formation, West Virginia

2019 AAPG Eastern Section Meeting:
Energy from the Heartland

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Organic Matter Macerals and Associated Mineralogical Matrices—Marcellus Formation, West Virginia


Despite the economic and scientific interest in organic matter (OM) storage in rocks, sediments and soils, mechanisms of preservation and degradation of this material in nature remain poorly understood. The fact that depositional environment and early diagenesis played important roles in the OM that we observe in ancient rocks today is well established. However, the great diversity of organic carbon in terms of host rock type, OM elemental composition, and style of OM preservation and degradation eludes simple explanations as to why, how, and how much organic carbon prevails over deep geological time. Eight sidewall core from the Marcellus Shale Energy and Environment Laboratory (MSEEL) science well MIP-3H were sampled and interrogated, from intervals spanning the Tully Limestone (shallowest interval, ~7200') through lower Marcellus (hydraulic fracturing target, ~7543') and Onondaga Limestone (deepest interval ~7554.5'). Among these core, mineralogy (as measured with Rietveld quantitative analysis of powder XRD data) and OM mineral matrices, (as measured with SEM BSE and QEMSCAN imagery) are diverse. Particular focus is on two Marcellus depth intervals: Marcellus top, ~7451' and lower Marcellus hydraulic fracturing target, ~7543', both of which are OM-rich yet very different in mineral composition and texture. Diagenetic replacement reactions, likely biologically mitigated, have resulted in partial mineralization of large OM (100s of µm in length scale) in these units by quartz, chamosite, pyrite, calcite, and dolomite, but the relative importance of these mineral replacements varies among samples. The smallest macerals resolvable with SEM (1s of µm to 10s of nm) are intercalated between clay- and silt-sized minerals and these contain the majority of pores. Comparing petrological properties of OM-rich intervals that are within the current hydraulic fracturing target zones to OM-rich intervals outside such zones contributes to an understanding of the properties that control economic viability of oil and gas plays. The OM macerals and their supporting rock matrices are distinctive for each core sampled, providing an opportunity to interpret high resolution imagery and microanalysis in concert with detailed geophysical and geochemical characterizations by other MSEEL researchers who have studied the same OM-bearing strata. The characterization of OM, associated mineralogy, pore features and overall rock fabric can be used to benchmark key downhole geophysical and geochemical measurements used to target optimum horizons.