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ABSTRACT

The three-dimensional Northern Gulf Coastal Province forms the arcuate northern half of the Gulf of Mexico Basin from the Rio Grande to southern Florida, between the Paleozoic-Mesozoic continental base and the Sigsbee Deep. The Mesozoic-Cenozoic deposits constituting the province vary from a feather edge to about 15,000 meters in thickness. They were deposited on the passive margin of southern North America after late Paleozoic-early Mesozoic rifting and separation from South America and Africa. The sedimentological history has been one of fluctuating Gulfward progradation—riftbasin fill, evaporitic calcareous-argillaceous-arenaceous-rudaceous early Mesozoic; later Mesozoic shales, marls, limestones, and chalks; and predominantly clastic deposition during the Cenozoic. Pulsations in rate of supply resulted from variations in rate and amount of supply, shifts in loci of deposition, cyclic glaciation, large scale tectonism, cyclic eustatic fluctuations, and thermal pulses. Volcanic and igneous materials are differentially distributed due to pulses of greater than normal heat flow during the late Cretaceous and early Tertiary. Beneath this mass is a collage of Paleozoic and Precambrian sedimentary, igneous, and metamorphic rocks.

The region has great structural variation dominated by extensional deformation. The regional dip is homoclinal away from the continent but is interrupted by numerous large and small positive and negative features.

Major source rocks are considered to be the late Jurassic, organically rich shales, marls, and limestones “trapped” in the rifted roots of the Northern Gulf Basin. In contrast, the source rocks of the Cenozoic were mainly deposited at the margins of the continent in a manner analogous to the organically rich Recent sediments identified by geochemists. Periods of maximum and accelerated maturation and migration resulted from episodic thermal pulses.

Reservoir rocks are principally shallow water in origin (fluviatile, deltaic, paralic, or shelfal) but may be bathyal. They are Triassic to Recent in age and vary from conglomeratic to fractured chalk and shale to salt domal caprock to fractured igneous rocks. Fracturing may be of much greater importance than heretofore recognized. Most significant reservoirs are arenaceous (90%). Abnormally high pressures appear to affect petroleum in carbonate reservoirs adversely.

Carbonate prospects generally possess fewer objectives than arenaceous prospects where the reservoir objectives are commonly multiply stacked. Almost all petroleum found to date has some relationahip to regional or local structural features, although some of the largest known accumulations are stratigraphically or paleogeomorphically controlled. No significant amounts of petroleum have been found continentward of the inner system of boundary faults. The Paleozoic basement reservoirs of the province have not been adequately explored.

Trap types span the full spectrum of structural, stratigraphic, paleogeomorphic, and combinational situations, e.g., fault traps, anticlinal and domal traps, porosity wedgeouts, and so on. About 80% of the known significant fields are primarily structural.

Petroleum is transitory in rocks unless entrapped (1) in a reservoir rock, and (2) in a trap with an effective seal. Evaporites, shales, chalks, and certain carbonates and fault-zone gouges are sufficiently abundant to have functioned extensively as seals in the region. The role of hydrodynamicism cannot be evaluated at this time.

Time of growth was clearly critical in entrapment as demonstrated by variations in amounts entrapped from (1) region to region, (2) salt dome to salt dome, (3) fault trap to fault trap, and (4) so on. In contrast, small quantities or the absence of petroleum in places may well be due to an absence of adequate volumes of sufficiently rich source rocks, insufficient thermal pulsing for maturation, or early expulsion of products.

Faults are predominantly normal (gravity, extensional, tensional); occur singly or in zones, mostly strike aligned; or are associated with other structural features and salt domes. Syndepositional faults commonly form functional “anticlinal or domal rollover traps” in the downthrown block. Petroleum occurs in fault traps in upthrown or downthrown blocks. No evidence is known of major strike slip displacement of the Mesozoic-Cenozoic strata.

Recoverable hydrocarbons discovered in the region to date are in the order of 125-150 billion barrels of liquid and liquid equivalents, about one half being liquid petroleum.

Petroleum has been found:

  1. in Paleozoic strata of the Black Warrior Basin, beneath the Mesozoic-Cenozoic coastal element;

  2. in sedimentary rocks of every age (Triassic?/Jurassic to Recent) which are an integral part of the coastal province (Fig. 6-12, 14-20);

  3. in Late Gulfian volcanic rocks emplaced in the sedimentary mass;

  4. from near the inner margin of the province to the vicinity of the continental slope;

  5. from mainly arenaceous (Fig. 18) but also from argillaceous and calcareous strata, salt-dome caprock, water-laid volcanics, altered igneous rocks, and fractured rocks of various kinds; and

  6. mostly in giant fields, i.e., 100 MM bbls of oil or equivalent (Fig. 14-20, 22).

Different reporting systems make it difficult to categorize production and significant fields by ages of reservoirs and production. As best we could tabulate and interpret the information, total liquids and liquid equivalents discovered through 1978 (in millions of bbls) are obviously incomplete and therefore on the low side:

We made a “best effort” analysis of average reserves per significant field by ages, discovered before 1976. Admittedly, they are incomplete and therefore low, but they may be of the right order of magnitude. In millions of bbls of L and LE, they are:

Ironically, in this age of modern computers, there is no publicly available, systematic, consistent summary of production as a result of variations in reporting, disagreements regarding age relationships, and so on.

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