Regional Stratigraphy and Reservoir Lithofacies Distribution of the Marble Falls Formation, North-Central Texas
The decline in natural gas prices in 2008 significantly impacted the exploitation of the Barnett Shale and eventually led to the discovery of a highly economic, fracture-driven, tight-oil resource play in the overlying Marble Falls Formation (MBLF) located along the western margin of the Fort Worth Basin (FWB). This study integrates core and petrophysical analysis, image log interpretations, and correlation of thousands of modern-day open-hole well logs to develop a regional stratigraphic framework of the MBLF from outcrop to the deep subsurface in an effort to evaluate its reservoir potential throughout the FWB.
The Early Pennsylvanian MBLF was first discovered in 1889 and has been studied extensively in outcrop and the immediate subsurface around the Llano Uplift where it was informally divided into lower and upper members separated by a regional unconformity that represents the Morrowan-Atokan boundary. In the core area of the current play in Jack and Palo Pinto counties the MBLF lies stratigraphically above the Comyn (Forestburg), or where the Comyn and Forestburg are absent, directly on top of the Barnett Shale. It is in turn overlain unconformably by siliciclastic deposits of the Atoka Group (“Bend Conglomerate”). Extensive well log correlation shows this thick sequence of the MBLF in Jack and Palo Pinto counties actually covers >15,000 square miles across the FWB and is part of the same Morrowan-age lower member of the MBLF outcropping along the periphery of the Llano Uplift. The formation gradually thickens eastward from its subcrop along the Bend Arch to more than 600 feet thick along the axis of the FWB to the east and ranges from 200 to 500 feet thick in the area of the present-day play.
The MBLF was deposited as part of a complex carbonate ramp system during the initial stages of Ouachita orogenesis and comprises an assortment of facies that vary vertically and laterally across the region. The position and variation in facies are typical of an inner ramp to basin depositional profile with a carbonate-rich environment along the western edge of the FWB to an increasingly more mudstone rich environment toward the east and northeast. There were a number of individual parasequences identified within the MBLF that generally show a shallowing upward profile from an outer ramp mudstone lithofacies to a more proximal spiculitic lithofacies and illustrate the influences of third- to fifth- order sea-level fluctuations and foreland basin tectonics.
Analysis of open-hole logs, petrophysical data, and side-wall core data show there is very little primary porosity and permeability in the MBLF. Further evaluation of whole cores and image log data indicate that the reservoir consists of a complex network of naturally occurring, lithology-bound fractures (LBF). The majority of these LBF are confined to the more brittle intervals containing an abundance of siliceous sponge spicules. In the northern part of the FWB, this spiculitic lithofacies constitute a large portion of the MBLF similar to that documented by previous workers around the Llano Uplift to the south. Understanding the distribution of various stratigraphic units within the MBLF and their control on reservoir quality will play an important role in the continued successful development of the modern MBLF play.
AAPG Datapages/Search and Discovery Article #90214 © 2015 Southwest Section AAPG Annual Convention, Wichita Falls, Texas, April 11-14, 2015