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Gas shales have become an important resource for the production of hydrocarbons in North America, and are being explored as a resource on other continents as well, based on their rapidly increased importance to the North American market and promise to boost domestic production elsewhere. There are numerous pricing and geopolitical reasons for this active exploration, but regardless of where in the world they are being explored, gas shales share some fundamental properties that make them virtually impossible to analyze with conventional core analysis methods or conventional petrophysical models based on log data. These properties are basically that gas shales are tight, with permeabilities in the 10s to 100s of nD, have low (effective) porosity, typically less than 10%, and have high kerogen and clay content. While there are some variations of these themes (e.g. shales with higher detrital input, making them siltier or silty-laminated), in general the tightness of the rock and abundance of clay minerals and kerogen pervades, and that causes a number of challenges to analysis. We have developed analytical methods for evaluating these reservoirs on core by using crushed material to enable better access to the pore space, retort analysis to measure separately free, bound and structural water saturations and also distinguish water from oil, and pressure transient analyses for the determination of permeability. Conventional core analyses (e.g. Dean Stark), applied to kerogen- and clay-rich rocks fails in separating free from bound waters and water from light oils, thereby missing critical inputs into calculating effective saturations, effective porosities and clay-bound water volume. In addition, the amount of oil recovered from retort, as an independent quantity, can be a useful proxy for kerogen maturity. From a permeability standpoint, unconventional reservoirs are usually too tight to allow for steady-state permeability measurements, and microfracturing is often too pervasive to allow for reliable permeability measurements on whole plug samples. As a result, we have developed a crushed-sample pressure decay system to measure the nD permeabilities typical of shales, and a stepped confinement pulse-decay method for measuring micro-Darcy (and higher) permeabilities in more texturally complex, siltier or sandier unconventional reservoirs that typically have these higher permeabilities.

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[1] ODP Site 1165, located 400 km northwest of Prydz Bay, contains a high-resolution early Miocene record of pulses of ice-rafted debris (IRD) originating from the ancestral Lambert Glacier system and the Antarctic coast to the east. The 520 m of early Miocene sediments consist of dark gray claystone with silt laminae (contourites) alternating with decimeter-scale layers of greenish-gray bioturbated claystone that commonly contain ice-rafted debris. Downhole logs also record the alternations between the two facies: the IRD-bearing greenish-grey claystone corresponds to high resistivity and density values because of increased cementation by silica, and corresponds to lower natural gamma radiation values because of diluted clay content. The downhole logs thus allow a continuous and detailed stratigraphic record of the IRD-bearing layers to be obtained. The IRD-bearing layers represent deglaciations and interglacials, when a high flux of icebergs with incorporated material were melting out over the site; the contourite-rich layers represent glacials, when the Polar Current over the site was relatively strong. The sequence is dated by magnetostratigraphy, and the timing of the major IRD pulses is paced by orbital eccentricity, indicating that the volume of the East Antarctic ice sheet also fluctuates on this timescale. After 19.7 Ma, minor precessional and subprecessional IRD layers appear in the record, indicating that the ice sheet becomes more prone to deglaciation through this interval, perhaps associated with the gradual warming trend through the early Miocene.