--> --> New Insights on the Regional Stratigraphy, Reservoir Characteristics, and Nomenclature History of the Marble Falls Formation, Fort Worth Basin

Southwest Section AAPG Annual Convention

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New Insights on the Regional Stratigraphy, Reservoir Characteristics, and Nomenclature History of the Marble Falls Formation, Fort Worth Basin

Abstract

The Early Pennsylvanian Marble Falls (MF) Formation in the Fort Worth Basin (FWB) was deposited in a broad carbonate ramp system during incipient Ouachita orogenesis. It covers an area of more than 15,000 square miles and contains an assortment of facies that vary considerably across the region due to high-frequency, glacio-eustatic sea-level fluctuations. The MF Formation has been studied extensively in outcrop around the periphery of the Llano Uplift and was informally divided into lower and upper members separated by a regional unconformity representing the Morrowan-Atokan boundary. However, only a few regional studies have been conducted on the Carboniferous stratigraphy in the deep subsurface of the FWB and most of those were published during the mid-1900's when well log data was limited. Consequently, the geological controls on the sequence stratigraphic architecture, facies patterns, thickness, and reservoir quality of the MF is still poorly understood. This study integrates data from core, petrophysical data, high resolution micro-resistivity image logs, and the regional correlation of >30,000 wireline logs to develop a comprehensive regional stratigraphic and paleogeographic framework for the lower MF from outcrop to the deep subsurface of the FWB. Understanding the stratigraphic controls on reservoir quality is important for evaluating reservoir potential of the lower MF in other areas of the basin and predicting future expansion of the play.

The lower MF was exploited conventionally over the last half century along the western edge of the FWB where operators targeted porous structural and stratigraphic traps or would use the MF as a bail-out zone when shallower formations were uneconomic. The MF also contains a highly porous and productive chert facies that formed near the top of the lower MF sequence when the carbonate ramp was subaerially exposed during a drop in sea-level at the Morrowan-Atokan boundary. Local operators have commonly referred to the lower MF using various terms like the “Duffer,” “Duffer lime,” “Comyn,” or “Rotten chert,” which has caused confusion with the nomenclature history and reporting of production and completion data. Public data sources show there are ∼1,720 “Duffer” or “Duffer Lime” wells, ∼40 “Comyn” wells, and ∼6,725 “Marble Falls” wells, the latter of which in most cases does not distinguish between the upper member (Atokan) and the lower member (Morrowan) of the MF Formation. Therefore, a regional historical production research was conducted and the completion and production data for each well in the FWB were manually compared to publically available wireline logs to determine the stratigraphic interval that was actually completed. The results show there are an estimated 3,600 conventional wells that have been completed within the lower MF interval in the FWB.

Recent exploitation of an unusually thick sequence (>400 ft) of the lower MF in the deep subsurface in the northern part of the FWB has developed into a highly economic, fractured-driven, tight-oil resource play. This sequence comprises high-frequency cycles that typically shallow upward from a basinal calcareous mudstone to an outer ramp spiculite facies which contains the best reservoir quality. The complicated lateral variations in these parasequences have compartmentalized the reservoir and controlled the distribution of reservoir lithofacies within the lower MF. Although the spiculite facies has very little primary porosity or permeability, the high concentrations of silica from the sponge spicules make it uniquely conducive to hosting a network of ubiquitous lithology-bound fractures (LBF) that constitute the reservoir.

In the core area of the current play in Jack and Palo Pinto counties the lower MF lies stratigraphically above the Forestburg-Comyn limestone or, where the Forestburg and Comyn are absent, directly on top of the Barnett Shale. It is overlain unconformably by siliciclastic deposits of the Atoka Group (“Bend Conglomerate”) in the northern part of the FWB and the upper MF limestone in the southern part of the basin. The formation gradually thickens from its subcrop along the western edge of the Bend Arch to more than 500 feet thick along the axis of the FWB to the east and ranges from 250 to 500 feet thick in the area of the present-day play. The regional distribution of the lower MF was tectonically controlled by the subsidence of the FWB and flexure of the Bend Arch (forebulge), but the thick, northwest trending section in the northern part of the FWB was stratigraphically controlled by several underlying Late Mississippian to Early Pennsylvanian units.

Extensive well log correlations show evidence that this lower Morrowan-age member of the MF Formation covers a much larger geographic area than has been previously estimated, and was found to be dominantly composed of the siliceous spiculite facies (reservoir lithofacies) across most of the FWB. The widespread distribution of this lower MF spiculite sequence suggests that the current play has tremendous potential for expansion.