Abstract We cannot hope to predict Mesozoic depositional processes and sediment properties well enough to plan effective regional exploration strategies without considering the big picture of Gulf of Mexico deposystem evolution. The two critical big picture elements are the kinematics and timing of the Yucatan Block's detachment and separation from North America and the various major expansions and contractions and the ultimate disappearance of the Western Interior Seaway. Although a number of authors, including this one, have speculated on the timing of separation of Yucatan from North America ( Fillon, 2007a ), no definitive evidence exists: i.e. , drilled samples of the ocean crust and the sediments directly overlying it. Without that unambiguous information we must infer the paleogeographic evolution of the early Gulf of Mexico Basin from deposystem architecture by asking questions such as when do Gulf of Mexico deposystems transition from architectures consistent with deposition in a youthful blockfaulted basin underlain by actively attenuating continental crust to deposition in a mature basin having stable margins surrounding a central region underlain by subsiding ocean crust. An understanding of the paleogeography and paleoceanography of the Gulf of Mexico Basin derived from deposystem architecture can help provide answers to crustal kinematic questions and to more exploration focused questions such as: where, and in section of what age should we look to find facies similar to the organic rich, generative Haynesville Shale facies of eastern Texas and western Louisiana. Although we all know something about the Western Interior Seaway, most of us working on the Mesozoic of the Gulf of Mexico Basin have not spent much time considering what effects it might have had on the prospectivity of Gulf of Mexico deposystems. Through much of Albian and Late Cretaceous time the Western Interior Seaway connected the Gulf of Mexico Basin with the Arctic Ocean Basin. The effects of the establishment and intermittent blocking of this major seaway connecting arctic and tropical water masses on global paleoceanography, on global paleoenvironments, and locally on onshore and offshore Gulf Basin deposystems cannot be ignored in our quest to understand the Mesozoic of the Gulf Rim. This paper is a “big picture” review of Gulf of Mexico Basin deposystem evolution within the Late Jurassic (Oxfordian)–Late Cretaceous (Maastrichtian) interval. Seventeen Mesozoic chronosequences are defined therein based on chronostratigraphic data garnered from over 130,000 industry well and pseudowell penetrations of Mesozoic section in the Gulf of Mexico Basin region. Examination of the collected data suggests that grouping the seventeen Gulf of Mexico Mesozoic chronosequences into seven super-chronose-quences optimally distinguishes key phases of deposystem and basin evolution. The oldest super-chronosequence defined in this study, dubbed “MG,” encompasses ca. 16.45 Ma of Norphlet through lowermost Cotton Valley Late Jurassic deposition. Sediment distribution and accumulation rates within the MG interval clearly define the rectilinear configuration of the earliest Gulf of Mexico Basin. This early basin geometry is consistent with fault controlled attenuation and foundering of North American continental crust, associated flooding, and rapid depositional infill concurrent with the earliest detachment of the Mayan (Yucatan) crustal block from North America. The Yucatan block, although showing an affinity with South American (Amazonian) terranes ( Martens, 2009 ), was left attached to the North American plate when North America began pulling away from Gondwana during the initial breakup of Pangea ( Fillon, 2007a ). The next younger super-chronosequence, “MF,” contains a. ca. 13.47 Ma record of Cotton Valley, Bossier, Knowles limestone., Late Tithonian through mid Hauterivian, deposition. The “MF” interval reflects the same rectilinear outline as the “MG,” but is marked by decreased accumulation rates, suggesting that the fault bounded crustal attenuation, rapid sediment infill phase had markedly slowed. The ca. 9.4 Ma of Hosston, Sligo, Sunniland limestone, James limestone, mid-Hauterivian through Early Aptian section contained within the succeeding “ME” super-chronosequence records modification of the early rectilinear basin outline by a temporary reactivation of attenuation and foundering in the western portion of the Gulf of Mexico Basin. “ME” sediment distribution patterns also indicate development of a depositional continental margin and accumulation of true continental margin type deltaic and reef systems. These observations suggest that during this interval a deep continental basin, probably floored by ocean crust, was beginning to form outboard of the attenuated continental crust. Sediment distribution and accumulation rates within the ca. 23.5 Ma Rodessa through lower Washita, Early Aptian through Early Cenomanian “MD” super-chronosequence reflect growth of the Wisconsin interior seaway and a stable phase of relatively low accumulation rates throughout the entire Gulf of Mexico Basin deposystem. During this interval, deposition was very likely influenced by a vigorous tidal and thermohaline current circulation driven by strong temperature contrasts within the Gulf of Mexico–Wisconsin interior seaway–Arctic Ocean connection. The next younger super-chronosequence, “MC,” contains a ca. ca. 16.0 Ma record of Dantzler, Washita, Lower Pine Key, Eutaw, Woodbine, Eagle Ford, Austin, and Early Cenomanian through Late Santonian (Late Cretaceous) deposition. During this phase, there is a marked reduction of accumulation rates in the north-western portion of the basin, attributable perhaps to expansion of the Western interior seaway and continued subsidence of the old Gulf of Mexico Basin margin. Associated small, perhaps tidal submarine delta-like depopods developed, perhaps in response to the regional Western interior seaway transgression ( Blakey, 2014 ). These delta-like depocenters appear to define a new basin margin presaging the modern curved shape of western Gulf of Mexico so familiar to us today. Here also we see the first unambiguous evidence of abyssal deposition in the deepest portion of the Gulf of Mexico Basin underlain by ocean crust. The succeeding ca. 12.82 Ma interval of Late Santonian through Early Maastrichtian upper Pine Key, upper Selma, upper Austin, Taylor, Olmos, Saratoga, and low accumulation rate mainly chalk and marl deposition contained within the “MB” super-chronosequence provides evidence of transgressive onlap associated with an expanding and deepening interior seaway during “MB” time. “MB” onlap has the effect of temporarily reemphasizing structural trends inherited from crustal attenuation that took place during “ME” time. Finally, the ca. 5.4 Ma long terminal Mesozoic “MA” super-chronosequence consists of Maastrichtian, Navarro equivalent, low accumulation rate marls deposited along the basin margin. These low accumulation rate basin rim sediments and low accumulation rate slope sediments are punctuated by high accumulation rate canyon fill and lobe-shaped slope depopods which are probably attributable to sediment reworking, transport and deposition by transitional Cretaceous-Paleogene (K/P) interval mega-tsunami backwash flows immediately following the Chicxulub impact. Higher accumulation rates in the deeper parts of the basin underlain by ocean crust are also consistent with high volume backwash flows.

A chronostratigraphic framework was developed for the subsurface Eagle Ford of South Texas in conjunction with a log-based regional study that was extended across the San Marcos Arch and into East Texas using biostratigraphic and geochemical data to constrain log correlations of 12 horizons from 1729 wells in South and East Texas. Seven regional depositional episodes were identified by the study. The clayrich Maness Shale was deposited during the Early Cenomanian in East Texas and northern South Texas where it correlates to the base of the Lower Eagle Ford. After a fall in sea-level, East Texas was dominated by the thick siliciclastics of the Woodbine Group, whereas in South Texas deposition of the organic-rich EGFD100 marls of the Lower Eagle Ford began during the subsequent Lewisville transgression. A shift in depositional style to the limestones and organic-rich shales of the Eagle Ford Group occurred in East Texas during the Middle-Late Cenomanian EGFD200 and EGFD300 episodes produced by the continued rise in sea-level. Erosion along the Sabine Uplift shifted the focus of deposition in East Texas southward to the Harris delta and deposited the “clay wedge” of northern South Texas during the EGFD400 episode. The introduction of an oxygenated bottom-water mass onto the Texas shelf produced the considerable decrease in TOC preservation that marks the Lower/Upper Eagle Ford contact. This event coincided with the onset of Oceanic Anoxic Event 2 (OAE2) and the Cenomanian-Turonian Boundary sea-level high, which starved much of the Texas shelf of sediment. The only significant source of sediment was from the south; within the study area, the EGFD500 interval is essentially absent north of the San Marcos Arch. Deposition recommenced on much of the Texas shelf during the Late Turonian EGFD600 episode with the Sub-Clarksville delta of East Texas and the carbonate-rich Langtry Member of South Texas and eastern West Texas. Bottom-waters became oxygenated at approximately 90 Ma, initiating the transition from the Eagle Ford Group to the Austin Chalk.

Abstract Twelve stratigraphic intervals originally defined in the Eagle Ford of south Texas were mapped across the San Marcos arch into the Maness Shale, Woodbine, and Eagle Ford of east Texas. The maps are based on well log correlations of 1729 wells across 22 counties in south and east Texas using biostratigraphic, geochemical, and lithologic data from 99 wells as seed points for the correlations. These mapped intervals were tied to a regional chronostratigraphic framework developed using data from the outcrops of west, central, and north Texas and cores from the subsurface of south and east Texas. Seven regional depositional episodes were identified across the Texas shelf for the Woodbine and Eagle Ford Groups based on the isopach maps, outcrop data, and paleoenvironmental interpretations. The clay-rich Maness Shale was deposited during the Early Cenomanian in east Texas and northern south Texas where it correlates to the base of the Lower Eagle Ford. After a relative fall in sea level, east Texas was dominated by the thick siliciclastics of the Woodbine, whereas in south Texas deposition of the organic-rich EGFD100 marls began during the subsequent transgression. A shift in depositional style to the limestones and organic-rich shales of the Eagle Ford occurred in east Texas during the Middle Cenomanian produced by the continued rise in sea level, correlating to the EGFD200 marls of south Texas and the carbonates of the Lozier Canyon Member (restricted) of the Eagle Ford Group in west Texas. During the EGFD300 interval deposition transitioned to the organic-rich marls and limestones of the Lozier Canyon and Antonio Creek Members of the Eagle Ford Group in west Texas and the Templeton delta became active in northern east Texas. Erosion along the Sabine uplift shifted the focus of deposition in east Texas southward to the Harris delta and deposited the “clay wedge” of the EGFD400 in northern south Texas. Although the lower part of the EGFD500 episode was deposited during OAE2, it is characterized by low total organic carbon (TOC) due to the presence of oxygenated bottom waters, and the Cenomanian–Turonian boundary sea-level high produced a regional hiatus. Deposition recommenced on much of the Texas shelf during the Late Turonian EGFD600 interval with the Sub-Clarksville delta of east Texas and the carbonate-rich Langtry of south Texas and eastern west Texas. Bottom waters became oxygenated at approximately 90 Ma, initiating the transition from the Eagle Ford to the Austin Chalk.

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.