Regional Stratigraphy of the Marble Falls Formation and Underlying Late Mississippian to Early Pennsylvanian Deposits, Fort Worth Basin, North-Central Texas, USA

Abstract

The Early Pennsylvanian Marble Falls Formation in the Fort Worth Basin (FWB) of North-Central Texas was deposited in a broad carbonate ramp system during the initial stages of Ouachita orogenesis. It extends across an area of more than 15,000 square miles and comprises an assortment of facies that vary considerably across the region due to high-frequency sea-level fluctuations. The Marble Falls has been studied extensively in outcrop around the periphery of the Llano Uplift where it was informally divided into lower and upper members separated by a regional unconformity representing the Morrowan-Atokan boundary. A sequence of the Marble Falls Formation was recently discovered in the northern part of the FWB where it reaches stratigraphic thicknesses of more than 450 feet and has become a fractured-driven, tight-oil resource play. The distribution of this unusually thick section was controlled by stratigraphic variations of several underlying units and extensive well log correlations across the FWB has presented new evidence that confirms this sequence is part of the lower Morrowan-age vi member of the Marble Falls Formation that covers a much larger geographic area than has been previously estimated. The examination of photomicrographs from whole cores and side-wall cores have also revealed that the lower Marble Falls is composed almost entirely of siliceous to partly calcareous sponge spicules similar to that observed in outcrop and this spiculite facies comprises most of the Marble Falls Formation across the entire FWB. This study integrates >20,000 open-hole well logs, micro-resistivity image logs, petrophysical data, and core data to better understand the regional stratigraphy of the lower Marble Falls and evaluate its reservoir potential throughout the FWB. It gradually thickens from its subcrop along the Bend Arch towards the axis of the FWB to the east and northeast and is characterized by an inner ramp to basin depositional profile. The influence of foreland basin tectonics and glacio-eustatic sea-level fluctuations are evident from stratigraphic trends and shallowing-upward parasequences that were identified within the lower Marble Falls. The complicated lateral variations in these high-frequency cycles have historically created high-porosity stratigraphic traps along the Bend Arch, but the present play is concentrated in an area where the lower Marble Falls has little primary porosity or permeability and the reservoir consists of a network of naturally occurring, lithology-bound fractures (LBF) that are concentrated in the more siliceous spiculitic lithofacies. Thorough evaluation of these high-frequency cycles and stratigraphic variations in the lower Marble Falls is critical in understanding the controls on reservoir quality and the distribution of reservoir lithofacies.