Abstract

In 2006, Mark Papa, CEO of EOG Resources, Inc. directed EOG divisions to focus on identifying and leasing large acreage blocks in shale oil window fairways (Mark Papa, personal communication) in basins throughout the United States while subordinating all exploration for natural gas, and in particular, dry gas. The company’s strategic change to shale oil exploration occurred during what was referred to as a “wall of disbelief” (Birger, 2011) predicated on the premise that oil molecules could not flow through shale-dominated permeability systems. The EOG Garner 1054 C#1, drilled in November 1998, encountered hydrocarbons within the Eagle Ford Formation at a pressure gradient of 0.76 psi/ft, at a subsurface true vertical depth (TVD) of 9300 ft (2834.6 m). Although a wet gas producer, this well was a critical element in the rationale to obtain leases in the oil window of the Eagle Ford Formation. Predicated upon a technical analysis of additional vertical well production within the Eagle Ford Formation indicating the existence of a dual porosity, or matrix-supported flow network, and in conjunction with the generation of fairway criteria mapping, EOG initiated a leasing strategy resulting in the acquisition of 569,000 contiguous acres within the crude oil window fairway.

Regional mapping of the Eagle Ford Formation was conducted to model structure, thickness, total organic carbon (TOC), thermal maturity (R_o), oil gravity, and hydrocarbon saturation as well as lithostratigraphic continuity, postulated environments of deposition, and mineralogical variations. An identified fairway situated between the Maverick Basin and the San Marcos arch, a syn-depositional graben system on the margin of a transgressed carbonate platform, was mapped as a relatively thick and laterally continuous stratigraphic section within the targeted R_o, TOC, and favorable hydrocarbon saturation windows. X-ray diffraction (XRD) analysis revealed that while the silica content within the Eagle Ford Shale was low relative to the more topical Barnett Shale and other existing shale resource plays, the mineralogical constituents were that of a brittle carbonate with variable clay content replete with a deceptive gamma log signature as a consequence of elevated levels of uranium and thorium.

Distinct structural settings displaying unique structural and stratigraphic attributes were recognized and mapped, all of which had remained within the oil generation window for the last 30 million years. Net rock volume increases associated with prolonged oil generation and expulsion were believed to increase the likelihood of catagenically induced micro-fracturing resulting in enhanced system permeability. Eight strategically located vertical delineation wells were drilled across a 15 by 120 mi (24.1 by 193.1 km) fairway located from Gonzales to LaSalle Counties. Conventional coring coupled with extensive electric logging suites and petrophysical evaluations provided an integrated regional understanding of the Eagle Ford Formation. Nanometer-scale imaging with focused ion beam (FIB) and field emission scanning electron microscopy (FESEM) of Eagle Ford core samples showed interconnected porosity systems and pore sizes capable of transmitting oil molecules. Initial production rates from EOG-operated horizontal delineation drilling confirmed the viability of the Eagle Ford Formation as an overpressured carbonate resource rock with system porosity and permeability capable of long-term economic oil production.

The methods defined in this chapter were appropriate for the delineation of the oil window within the Eagle Ford of South Texas; however, hydrocarbon systems are unique and these methods may not be applicable for defining other plays within other basins.