Atoka Sands (Pregnant Shale or Davis Sand) Play
Geology
Collins, et al published a detailed description of the interval in 1992 (in Depositional Environments, Petrology and Fractures of the Atoka Davis Sandtone: A Low-permeability Gas-bearing Sandstone of the Fort Worth Basin, North Central Texas {Transactions of the Southwest Section of American Association of Petroleum Geologists Annual Convention, Publication SWS 92-90}). Much of the work focused on the natural fractures occurring in the horizon. Today, the interpretation of the depositional environment has changed, but the overall theme is still the same: that of a low porosity and low permeability reservoir covering a large area, with natural fractures cutting both hard dense black shale and thin fine grain sand lenses. Core was obtained for the study and pictures of these cores are provided in Photos 1 through 4. Core from the HSFT #1 research well and the Sallie Hill #1 were compared to core (the productive Mitchell Energy Sims #1) from the prolific and currently active Barnett Shale.
The Pregnant Shale, also known as the Davis Sand, and the Grant Sand at the base of the Pregnant Shale, are included in the geologic interval known as the Smithwick Shale Group. The Pregnant Shale gets its name form the characteristic pattern of resistivity seen on many logs for the interval. The typical sequence begins at the base with the 25 to 50 feet of clean “blocky” pattern seen in the Grant Sand Member (see Map 2). The Pregnant Shale then grades upwards into higher resistivities and courser sands, forming a tornado shape, or characteristic “pregnant” shape on the log curves; hence, the Pregnant Shale was named for the pattern seen on resistivity logs.
The higher resistivity Pregnant Shale gives way to low resistivity black “poker chip” shale above. This shale has a very different appearance than the Pregnant Shale, tending to be softer, more friable, and has strong water absorbing characteristics. No fractures occur in the shale above the Pregnant, whereas the Pregnant Shale tends to be fractured, in both the hard black shale and the thin fine grain sands. Fractures are important in that they originally enabled the movement of natural gas throughout the formation and after the formation is fraced enable the reservoir to be drained.
The Grant Sand is interpreted to be deposited as a basin-floor fan in fairly deep water, during the early stages of filling the Fort Worth Basin. The basin-floor fan was then covered by the deep water turbidite fans of the Pregnant Shale/Davis Sand. The turbidite fans gave way to the pro-delta shales of the following deltaic sequence; first with soft black shale, and later sands and coal deposited in a widespread delta (Map 2).
Some example photos can serve to show how the Pregnant Shale differs from the overlying Caddo age pro-delta shale, and how fractures cut through both shale and sandstone lenses.
Photo 1 shows the typical Pregnant Shale/Davis Sand rock, with thin sand lenses intermingled with dense hard black shale. The two rock types are inter-fingered and laminated, but hold together as one solid core section over one foot long. The thickness of the sand lenses is less than one inch, far too thin to be resolved by most electrical logs, even though sand makes up about 50% of the volume of core.
Photo 2 shows natural fractures in the Pregnant Shale. Contrast this with Photo 3 of the Barnett Shale. The Barnett has no thin laminations of sand, but like the Pregnant Shale, it has fractures in hard, dense, black shale that is brittle in nature.
Photo 4 compares the Pregnant Shale to the Barnett Shale. Photo 5 compares the Pregnant Shale to the overlying soft, black “poker chip” shale in the same well (HSFT #1).
Photo 6 shows an enlarged view of the fractured Pregnant Shale/Davis Sand from the HSFT #1 well (exact depth is unknown). The fractures cut across both the sand and the shale lenses. On the same rock sample, turned to view the plan view of the core piece (Photo 7), the fractures are seen to cut across both sand and shale. Although this core sample has been handled before, the sample appears to show an open aperture along the fracture plane, which would suggest that the fractures may be open in some places.