The Gulf of Mexico continues to be a premier basin for global hydrocarbon exploration and development. An over whelming volume of information has been generated by decades of onshore and offshore seismic acquisition and drilling. A major challenge facing explorationists is the synthesis of this information into a framework that can focus efforts in the current deepwater slope and basin sand fairways.
The Gulf Basin Depositional Synthesis project (GBDS) has applied global information system (GIS) technology to create an interpretive database that combines information from more than 800 wells, 300 publications, and 41,000 km of deep-basin seismic lines. Data are grouped into 18 major sequences, or depositional units, that compose the Cenozoic (past 65 million years) sedimentary fill of the basin.
Practical considerations required use of genetic sequences that are bounded by regionally correlative and datable marine transgressions. Transgressive boundaries of sequences are readily correlated from basin center to basin margin using both well and seismic data. Genetic sequences record episodes of sediment supply and filling of the Gulf basin, and commonly reflect periods of relatively stable basin geography, patterns of sand transport, and patterns of sand deposition.
For each sequence, well and shotpoint specific information and representative maps were prepared and stored as part of the data base. Information and interpretive displays for each sequence include:
- Depth of sequence top
- Sediment facies associations penetrated by wells or imaged at seismic shotpoints
- Distribution and thickness of net sand-bearing interval
- Depositional systems and paleogeography
- Distribution of important geologic features such as shelf edges, submarine canyons, and reefs
- Axes of principal sediment transport.
A major task of the project was the correlation of basin margin sequences, largely known from the thousands of wells that penetrate their deposits, with the sequences of the deep basin, which are identified and mapped using seismic data. Once correlations were defined and tested, sequences could be mapped from basin margin to basin floor.
Geographic features identified and mapped included rivers, deltas, shore zones, shelves, various kinds of continental slopes, and basin floor systems. In all, 25 different kinds of depositional systems were differentiated. Know ledge of major sediment supply routes through marginal rivers and delta systems aided prediction of sand dispersal paths down the slope and onto the basin floor.
Upper Miocene example
The Upper Miocene (UM) sequence is a major deep slope and subsalt exploration target and illustrates the kinds of interpretive maps that were prepared. Such maps provide a framework for local prospect development and reservoir evaluation. The map of total UM thickness shows the thickest deposits to lie beneath the inner shelf off southeast Louisiana. Here a large paleo-Mississippi delta system, which was fed by two adjacent large rivers, built onto the Gulf margin.
The southward bulge of the contemporary shelf edge, shown in dark blue, reflects the construction of new continental shelf beyond its earlier position (shown by the dashed line). The delta expanded to the shelf edge, constructing the continental slope by prograding a sandy apron of deepwater deposits southward. Sand spilled from the delta margins, down slope, where much was trapped in structurally active intraslope basins that filled with apron sediment.
The belt of thickest UM section, 10,000-12,000 ft thick, records accumulation of the thick delta front and slope apron sediment wedge required to construct the new continental margin. Significant quantities of sand also bypassed the slope and settled onto the central Gulf floor, to complete the sandy Central Gulf Apron.
Of particular interest is the area in the northeast Gulf where UM thickness contours bulge basinward to form a radiating network of sandy sediment thicks that project across the basin floor. The radial distribution pattern, basin-floor thick, and seismic attributes of the UM equivalent seismic sequences (which suggest sandy turbidite facies) all confirm that a large submarine fan (designated the McAVLU fan for the three OCS areas that it lies beneath) extended from the continental slope and onto the Gulf floor.
Compiled seismic mapping shows a large UM thick located offshore of the modern Mississippi delta, beneath the Mississippi Canyon OCS area, confirming the GBDS map. The very sandy UM delta of the ancient eastern Mississippi river supplied sediment across the shelf margin to create a comparably sand-rich submarine fan that, in turn, contains thick productive reservoirs. The map illustrates how linkages between depositional systems can be traced by regional mapping of the Gulf margin. Such linkages provide important guides to likely sites of deepwater reservoir development. Recognition of sand-prone seismic facies on the adjacent basin floor confirms that reservoirs are likely to extend through the intervening slope deposits.
To the west, a relatively thinner succession of wave-reworked beach and barrier island deposits form a shore-zone complex that extends from western Louisiana along the Texas coast and inner shelf. Several small rivers and deltas occurred along this shoreline. Modest oil and gas production occurs in sand bodies of these systems.
Deep basin prospects appear less promising, however, as a muddy shelf generally separated the sandy shore-zone from the adjacent slope. Only in the western Gulf, where the shelf was narrow and tectonic uplands close by, are sandy UM deposits indicated by basin floor seismic response.
Deep Gulf reservoir potential
Reviewing the parade of paleogeographies illustrated by the maps of the GBDS synthesis reveals three generalizations about Gulf Basin depositional history, evolving patterns of sediment supply, and reservoir development.
- The Cenozoic history of basin filling is summarized by the pattern of shelf edge migration. The northern and northwestern margins have prograded 150-180 miles from their inherited Cretaceous position. Each of the major depositional episodes prograded the shelf edge along many hundreds of miles of the Gulf margin. Shelf outbuilding has only locally and briefly been interrupted by abrupt subsidence, due to salt withdrawal and/or by submarine mass wasting. Out building of the shelf began relatively uniformly at first, but by Late Miocene time, had become focused in the north-central Gulf. The western margin, in contrast, has been displaced largely by tectonic uplift or subsidence. Slope bypassing has dominated, commonly creating sandy basin floor turbidite aprons, as seen in the UM sequence. Mar gins advanced most rapidly in front of the largest deltas, where sediment supply to the basin was greatest.
- Immense volumes of sand have bypassed the shelf margin to be deposited in slope and base-of-slope systems during all major Paleo gene and Neogene depositional episodes. Thick potential reservoir sand bodies occur in offlapping delta-fed slope and basin aprons, in destructional slope aprons infilling slide scars and submarine canyons, and in submarine fans. Sand bodies traverse the slope depositional wedge and commonly extend many tens of miles across the basin floor; however, the greatest sand volume is entombed within outbuilding delta-fed aprons. Principal reservoir facies are turbidite channel fills, ponded turbidite lobes (commonly called sheet turbidites) within intraslope salt basins, and unconfined turbidite lobes on the basin floor. Sandy slump and slide deposits also constitute local reservoir sand bodies in the aprons. Basin floor submarine fan systems also contain thick successions of sandy turbidite channel and lobe facies. In fan systems, potential reservoir sand bodies are vertically stacked, laterally extensive, and project more than 100 miles across the basin floor. Their basin floor location places large areas of older fan systems beneath young salt tongues, as well as basinward of the present slope toe.
- Fans have developed several times in the Gulfs history. Three fans developed in the east ern Gulf, beginning with deposition of the Middle Miocene sequence. The modern Mississippi fan is only the youngest of these fans. However, earlier fans were not related to large, equally long-lived submarine canyons that eroded onto the shelf. Only the Pleistocene canyons of the Mississippi fan system have been mapped more than a few miles landward of their paleo-shelf edge into contemporary shelf and coastal deposits. Older fans connect to sandy dispersal axes within depositional aprons or to clusters of small canyons and slump scars. Fan system origin appears to be most commonly tied to development of continental margin collapse and sediment failures. Where developed, such large basin floor fan systems are prime reservoir targets. They record areas of focused, long-lived sand bypass from the shelf to the base of the slope. Furthermore, most Gulf fan systems originate along the margins of large delta systems where wave reworking has removed much of the fluvial mud and improved sand sorting and percentage.
GBDS maps have delineated the regional reservoir potential of the deep Gulf Basin. Sand bodies are widely developed in slope and adjacent basin floor deposits of most sequences. Under standing the paleogeography and sediment dispersal systems that prevailed during each of the sequence-forming depositional episodes quickly guides the explorationist to the areas and stratigraphic intervals likely to contain the best reservoir potential. Such spatially defined information is efficiently compiled, synthesized, interpreted, and displayed using a GIS.
The Gulf Basin Depositional Synthesis project is an industry-supported consortium at the Institute for Geophysics of The University of Texas at Austin. The contributing members are AGIP, Amoco, Anadarko, BHP, Burlington, Conoco, Elf, Exxon, Marathon, Mobil, Norsk Hydro, Oryx, Pemex, Phillips, Texaco, Total, and Vastar. The project uses ArcInfotrademark and ArcViewtrademark software to create a map-based suite of data tables and paleogeographic features that summarize rock body distribution and depositional history. Additional information can be obtained at [email protected].