From Oil-Prone Source Rock to Gas-Producing Shale Reservoir – Geologic and Petrophysical Characterization of Unconventional Shale-Gas Reservoirs

Abstract

Many currently producing shale-gas reservoirs are overmature oil-prone source rocks. Through burial and heating these reservoirs evolve from organic-matter-rich mud deposited in marine, lacustrine, or swamp environments. Key characterization parameters are: total organic carbon (TOC), maturity level (vitrinite reflectance), mineralogy, thickness, and organic matter type. Hydrogento-carbon (HI) and oxygen-to-carbon (OI) ratios are used to classify organic matter that ranges from oil-prone algal and herbaceous to gas-prone woody/coaly material.Although organic-matter-rich intervals can be hundreds of meters thick, vertical variability in TOC is high (<1-3 meters) and is controlled by stratigraphic and biotic factors. In general, the fundamental geologic building block of shale-gas reservoirs is the parasequence, and commonly 10's to 100's of parasequences comprise the organic-rich formation whose lateral continuity can be estimated using techniques and models developed for source rocks.Typical analysis techniques for shale-gas reservoir rocks include: TOC, X-ray diffraction, adsorbed/canister gas, vitrinite reflectance, detailed core and thin-section descriptions, porosity, permeability, fluid saturation, and optical and electron microscopy. These sample-based results are combined with full well-log suites, including high resolution density and resistivity logs and borehole images, to fully characterize these formations. Porosity, fluid saturation, and permeability derived from core can be tied to log response; however, several studies have shown that the results obtained from different core analysis laboratories can vary significantly, reflecting differences in analytical technique, differences in definitions of fundamental rock and fluid properties, or the millimeter-scale variability common in mudstones that make it problematic to select multiple samples with identical attributes.Porosity determination in shale-gas mudstones is complicated by very small pore sizes and, thus, large surface area (and associated surface water); moreover, smectitic clays that are commonly present in mud have interlayer water, but this clay family tends to be minimized in high maturity formations due to illitization. Finally, SEM images of ion-beam-milled samples reveal a separate nanoporosity system contained within the organic matter, possibly comprising >50% of the total porosity, and these pores may be hydrocarbon wet, at least during most of the thermal maturation process. A full understanding of the relation of porosity and gas content will result in development of optimized processes for hydrocarbon recovery in shale-gas reservoirs.

Multiple-stacked Hybrid Plays of lacustrine source rock intervals: Case studies from lacustrine basins in China

“…In recent decades, most organic-rich fine-grained tight plays, e.g., Barnett, Niobrara, Eagle Ford, and Bakken have been labeled ''shale plays'' and the terms of ''shale oil,'' ''tight oil,'' and ''resource play'' are often used interchangeably in public discourse (Curtis 2002;Bustin 2006;Passey et al 2010;Jarvie 2012a;EIA 2013). In reality, the shale interval is only a subset of all low permeability finegrained tight rocks in the source rock interval, including sandstone, siltstone, carbonates and shale.…”

“…This usage has created a certain amount of ambiguity in academia, researchers, and industry regarding the meaning of these terminologies since they are either too broad or too narrow. The definition of ''shale play or shale reservoir'' is a continuous fine-grained tight plays associated with organic-rich shale source rock that is also a reservoir (Curtis 2002;Bustin 2006;Passey et al 2010). The lithologic definition of shale is a rock consisting of extremely fine-grained particles to variable amounts of silt-size particles with a wide range in mineral composition (clay, quartz, feldspar, heavy minerals, etc.)…”

“…''Continuous accumulation'' was mainly used to characterize the reservoirs charged with continuous hydrocarbons, e.g., basincentered tight gas (Schmoker 1995); it does not define the types of unconventional reservoirs. ''Unconventional reservoir/play'' is a broadly generic term that has been used by industry for a wide range of play types including ''coal bed methane (CBM)'' ''tight gas,'' ''oil shale,'' ''shale gas,'' ''shale oil,'' ''tight oil,'' ''halo oil,'' and others (Passey et al 2010;Clarkson and Pedersen 2011). This usage has created a certain amount of ambiguity in academia, researchers, and industry regarding the meaning of these terminologies since they are either too broad or too narrow.…”

“…The lithologic definition of shale is a rock consisting of extremely fine-grained particles to variable amounts of silt-size particles with a wide range in mineral composition (clay, quartz, feldspar, heavy minerals, etc.) (Passey et al 2010). ''Hybrid shale-oil resource system'' defines all fine-grained organic-rich and organic-lean plays as a system where hydrocarbons are produced from both organic-rich shales and juxtaposed organic-lean lithofacies (Jarvie 2012a, b).…”

Geochemical and lithological controls on a potential shale reservoir: Carboniferous Holywell Shale, Wales

“…These factors relate in turn to the nature and amount of organic matter in the rocks, and also their lithology and mineralogy (e.g. Passey et al, 2010). In a depositional system like the Holywell Shale, in which both water depth and the supply of both sediment and organic matter changed over short periods of geological time, controls on shale reservoir quality will be complex in both space and time, and thus difficult to predict.…”