Stratigraphic architecture, depositional systems, and lithofacies of the Mississippian upper Barnett Two-Finger Sand, Midland Basin, Texas

The upper Barnett Two Finger Sand in West Texas forms oil and gas reservoirs that produce from natural fractures and low-permeability intraparticle matrix nano- to micropores. The section contains two mixed silt- to very fine sand-sized siliciclastic‒carbonate units that are composed of relatively thin 0.5 mm to 1 m hybrid/cogenetic event beds that amalgamate to form a deepwater submarine fan system in the Mississippian Tobosa Basin. Siliceous mudstones occur between these coarser grained units. The sediment sources for the fan lobes were from the northern and eastern margins (inner ramp) of the basin. The depositional setting, based on elevated TOC (up to 3.1%), lack of wave-related hydrodynamic features, and deepwater biota (cephalopods, Archeodiscus sp. foraminifera), is interpreted as having been below storm-wave base in a generally dysaerobic bottom-water setting with brief periods of oxygenation. Most of the TOC is Type III, but minor amount of Type II is present. Calculated Ro values average 1.2%, situating it in the late oil to early gas window. Core-plug porosity ranges between 0.5 to 4.4% and all permeability analyses were less than 0.001 md. Pore types include intraparticle fluid-inclusion pores and clay-platelet mineral pores. Some microvugs (<1 µm to 10 µm), that may be related to dissolution are observed. Rare organic-matter pores are present. Based on decline curves from producing wells and shut-in-pressure tests performed during the development of the Moonlight Field, it was concluded that the permeability network in the wells are natural fractures. After a few years of production, however, the decline curves suggest that some production is from the matrix. The concepts developed concerning the depositional setting and sediment-source areas and directions may aid in extending the development of the Two Finger Sand.