Geologic assessment of undiscovered oil and gas resources in Aptian carbonates, onshore northern Gulf of Mexico Basin, United States Paul C. Hackley * , Alexander W. Karlsen U.S. Geological Survey, MS 956 National Center,12201 Sunrise Valley Dr., Reston, VA 20192, USA article info Article history: Received 12 September 2013 Accepted in revised form 21 December 2013 Available online 20 February 2014 Keywords: Sligo Formation James Limestone Hydrocarbon resource assessment Lower Cretaceous Gulf of Mexico Basin abstract Carbonate lithofacies of the Lower Cretaceous Sligo Formation and James Limestone were regionally evaluated using established U.S. Geological Survey (USGS) assessment methodology for undiscovered conventional hydrocarbon resources. The assessed area is within the Upper JurassiceCretaceouseTer- tiary Composite total petroleum system, which was defined for the assessment. Hydrocarbons reser- voired in carbonate platform Sligo-James oil and gas accumulations are interpreted to originate primarily from the Jurassic Smackover Formation. Emplacement of hydrocarbons occurred via vertical migration along fault systems; long-range lateral migration also may have occurred in some locations. Primary reservoir facies include porous patch reefs developed over paleostructural salt highs, carbonate shoals, and stacked linear reefs at the carbonate shelf margin. Hydrocarbon traps dominantly are combination structural-stratigraphic. Sealing lithologies include micrite, calcareous shale, and argillaceous lime mudstone. A geologic model, supported by discovery history analysis of petroleum geology data, was used to define a single regional assessment unit (AU) for conventional reservoirs in carbonate facies of the Sligo Formation and James Limestone. The AU is formally entitled Sligo-James Carbonate Platform Oil and Gas (50490121). A fully risked mean undiscovered technically recoverable resource in the AU of 50 million barrels of oil (MMBO), 791 billion cubic feet of natural gas (BCFG), and 26 million barrels of natural gas liquids was estimated. Substantial new development through horizontal drilling has occurred since the time of this assessment (2010), resulting in cumulative production of >200 BCFG and >1 MMBO. Published by Elsevier Ltd. 1. Introduction 1.1. Assessment background The U.S. Geological Survey (USGS) recently conducted a regional assessment of undiscovered hydrocarbon resources occurring in onshore Mesozoic strata in the north-central Gulf of Mexico Basin (Dubiel et al., 2010, 2011). This work was mandated as part of the USGS mission to provide information that can be used for planning strategic use of fossil energy and protection of the economic health of the nation. In this regard, the USGS is continually assessing un- discovered oil and gas resources of priority basins within the US to update the comprehensive 1995 national assessment of oil and gas resources (Gautier et al., 1996). The purpose of this report is to present results of geologic assessment of undiscovered conventional oil and gas resources occurring in carbonate facies of the Lower Cretaceous Sligo Formation (including the Pettet Formation), and James Limestone in the onshore United States part of the Gulf of Mexico Basin. Clastic facies of the Sligo Formation were separately assessed (Doolan and Karlsen, 2011). The area of the Gulf of Mexico Basin evaluated in this report includes petroleum provinces that were previously assessed in 1995: the Western Gulf (Schenk and Viger, 1996a), East Texas (Schenk and Viger, 1996b), and the LouisianaeMississippi Salt Ba- sins (Schenk and Viger, 1996b). The study area extends northeast- ward from the border with Mexico on the southwest, through south-central Texas and eastward into Louisiana, Mississippi, and Alabama (Fig. 1). The assessment was conducted using established methodology of the USGS National Oil and Gas Assessment Project (e.g., Klett et al., 2005; Charpentier and Klett, 2005), in conjunction with literature review, evaluation of proprietary oil and gas production and well databases (IHS Energy, 2009a, 2009b), and discovery history analysis of oil and gas field and reservoir databases (Nehring Associates, 2009). The USGS methodology employed for the Gulf of Mexico assessment did not consider hydrocarbon accumulations * Corresponding author. Tel.: þ1 7036486458. E-mail address: phackley@usgs.gov (P.C. Hackley). Contents lists available at ScienceDirect Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes 0195-6671/$ e see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.cretres.2013.12.005 Cretaceous Research 48 (2014) 225e234