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The AAPG/Datapages Combined Publications Database

Rocky Mountain Association of Geologists

Abstract


Gas Shale in the Rocky Mountains and Beyond, 2008
Pages 85-117

Chapter Three: Origin, Conditions, and Timing of Gas Generation in the Lewis Shale, San Juan Basin, New Mexico

Neil S. Fishman, Thomas M. Parris, Donald L. Hall, Paul G. Lillis, Mark J. Pawlewicz

Abstract

The Upper Cretaceous (Campanian) Lewis Shale, San Juan Basin, northwestern New Mexico and southwestern Colorado, which is one of several marine units deposited in the Western Interior Seaway, produces natural gas from reservoirs that were previously considered “bypass” zones in wells targeting deeper Cretaceous reservoirs. Organic petrology and geochemistry, petrography, stable isotope, and fluid inclusion studies were conducted on subsurface core and outcrop samples from around the San Juan Basin to investigate the origin of the natural gas in the Lewis. Lewis reservoirs are typically fine grained (mudstone, siltstone, very fine grained sandstone) with low porosity and permeability. Total organic carbon varies from 0.6 to 3.26 wt %. Gas-prone terrestrial organic matter (type III kerogen) is dominant in the formation with lesser amounts of marine organic matter (type II kerogen). Maturity of the organic matter varies, with mean vitrinite reflectance values ranging from ~1.5% near the basin axis to about 0.8% toward the basin margins. The reservoir and organic geochemical characteristics thus strongly suggest that natural gas in the Lewis was self-sourced. Moreover, our studies further suggest that multiple mechanisms acted to generate gas in the Lewis.

Bacterially mediated methanogenesis (dry gas) took place during Lewis deposition and shallow burial and was the first gas generation mechanism acting in the formation. Precipitation of concretions occurred during methanogenesis, with concretionary calcite in some having an isotopic signature that suggests it precipitated from marine and/or modified marine pore fluids. Possible primary methane-bearing fluid inclusions in concretionary minerals points to coeval methanogenesis and concretion growth, and the widespread distribution of concretions in the basin implies that methanogenesis took place over a broad area.

Subsequent generation of wet, thermogenic gas and oil probably commenced when the Lewis reached sufficient burial temperatures. Possible primary single-phase inclusions in quartz fracture cement that contain methane, ethane, and propane, as determined by fluid inclusion microthermometry and Raman spectrometry, points to fracture cementation as temporally related to generation of wet gas. In very fine-grained sandstone intervals, syntaxial quartz overgrowths and later intergranular calcite cement contain blue fluorescent high gravity oil inclusions of possible primary origin. Aqueous homogenization temperatures for possible primary inclusions in syntaxial quartz (106-114°C) suggest thermal conditions were sufficient to generate thermogenic gas and oil during precipitation of quartz. Further evidence includes non-fluorescing dead oil in fractures and interstitial pores in some thin, very fine grained sandstones or siltstones interbedded with mudstone/shale units. The oil is likely to have been generated from the type II organic matter in the Lewis.

High paleotemperatures (155-170°C) on core samples from near the basin axis, as inferred from vitrinite reflectance (mean Ro = 1.2 to 1.5%) and some fluid inclusion data (possible primary inclusions and inclusions having an unknown origin in quartz with hotter and more variable homogenization temperatures), indicate that sufficiently hot thermal conditions may have existed, locally, in the Lewis to have resulted in dry gas generation from kerogen as well as through the cracking of oil. Assuming that at least some dry gas in the deepest part of the basin was generated through the cracking of oil, then a third mechanism may have acted in the Lewis to generate gas.

Data generated in this study were examined in the context of previous burial history modeling, and the analyses indicate that petroleum generation in the Lewis spanned much of its burial history, starting at the time of deposition (Campanian) and continuing for at least 56 m.y. through maximum burial, although it could have continued until the Pliocene, when the Lewis cooled as a result of uplift. Intraformational fine grained intervals (e.g., mudstones, muddy siltstones) and possibly the clay-rich Huerfanito Bentonite Bed, which is interbedded with the Lewis, promoted retention of gas and served as internal seals through geologic time.


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