Charles D. Winker

Paleogeographic Evolution of Early Deep-Water Gulf of Mexico and Margins, Jurassic to Middle Cretaceous (Comanchean)

The paleobathymetric configuration of the early Gulf of Mexico is inferred from (1) Cretaceous carbonate shelf margins interpreted from seismic profiles and other stratigraphic data; (2) distribution of Jurassic and Cretaceous platform and basinal facies; and (3) hindcasting of subsidence history in the central basin. Substantial paleogeographic ambiguity results from uncertainty about (1) kinematics and timing of Late Triassic to Jurassic extensional opening of the Gulf basin, which probably involved major strike-slip faulting, (2) the magnitude of subsequent compressive deformation on the western and southern basin margins, and (3) possible accretion of allochthonous terranes. The initial sub-sea level topographic depression created by crustal extension is manifested by the distribution of Callovian (?) pre-marine salt and Oxfordian basinal marine facies. Late Jurassic sea-floor spreading split the main salt body and created a central deep-water (> 1 km) trough. From the Oxfordian to early Neocomian, ramp-like platform margins generally underwent net retreat, except in the northern Gulf, where major clastic progradation took place. Carbonate platform margins with sufficient paleorelief for geometric expression on seismic profiles developed early in the Cretaceous. These Cretaceous carbonate margins exhibit a variety of architectural styles, including (1) in the northwestern Gulf, two cycles of progradation (Coahuilan and Comanchean) terminated by drowning events; (2) in the southwestern Gulf, a single cycle of progradation; (3) continuous aggradation along the Florida and Campeche Escarpments. Paleobathymetric relief across Cretaceous carbonate margins ranges from high and steep, to low-relief, to ramplike. The close-of-Comanchean drowning event greatly and permanently reduced the extent of circum-Gulf carbonate platforms and especially rimmed margins. Locations of Cretaceous carbonate margins were influenced by basement hinge zones and paleohighs developed on attenuated continental crust, by the distribution of salt, and by clastic progradation. Eustacy is commonly invoked to explain behavior and architecture of carbonate margins in the Gulf, but other paleoenvironmental variables such as nutrient abundance are probably important as well.

Seismic geomorphology, lithology, and evolution of the late Pleistocene Mars-Ursa turbidite region, Mississippi Canyon area, northern Gulf of Mexico

The interplay between sedimentation and erosion during the late Pleistocene in the Mars-Ursa region, northern Gulf of Mexico, resulted in a complex compartmentalized reservoir. Rapid deposition, directly downdip of the Mississippi River beginning about 70 k.y., quickly filled antecedent topography in the Mars-Ursa region with a thick accumulation of sand and mud called the blue unit. This permeable reservoir was rapidly and asymmetrically buried by thick, mud-rich levees of two channel-levee systems. Both systems plunged from north to south with a steeper gradient than the underlying blue unit. Rotated channel-margin slides present in both channel-levee systems rotated low-permeability, mud-rich levee deposits beneath the sand-rich channel fill. As a result of the channel-levee systems, the east-west hydraulic connectivity of the blue unit decreases progressively from north to south until its eastern and western halves become completely separated.

High-Resolution Seismic Stratigraphy of a Late Pleistocene Submarine Fan Ponded by Salt-Withdrawal Mini-Basins on the Gulf of Mexico Continental Slope

The late Pleistocene Brazes-Trinity Fan, a structurally ponded fan completely exposed and undisturbed on the seafloor, was mapped with a combination of cmventional and high-resolution seismic data, This fan occupies three salt-withdrawal minibasins (I, II, IV) and a gratwm (111),each filled with an onlapping package consisting of alternating bedded and nonbedded units evident on high-resolution data. Basins 1-111are filled to their topographic spill point.%the ordap-fill succession of each is incised by a channel system which bypassed sediment to the next basin(s) downdip. Seismic continuity generally incmses distally in the system and within individual basins, believed to reflect the increasing prevalence of turbidity currents over high-density sediment gravity flows. Introduction A late Pleistocene submarine fan system linked to a shelfmargin delta of the Brazes and Trinity Rivers (Fig. 1) was ponded on the upper Texas continental slope during the last glacio-eustatic lowstand of sea level by three salt-withdrawal mini-basins (Fig. 2, 3). Such ponded fan systems are abundant in the subsurface of the Gulf of Mexico continental slope, but only the Brazes-Trinity fan is still exposed intact on the seafloor. Shell Development Company and Shell Offshore, Inc. conducted a high-resolution seismic stratigraphic study of the Brazes-Trinity fan to provide an analog for more deeply buried and less well-imaged ponded fans. The Brazes-Trinity fan was previously recognized by Gardiner’ and Satteti]eld and Behrens2on the basis of seafloor fan channels seen on seismic profiles. Earlier studies34 had depicted this system simply as chaotic fill, similar to the chaotic facies comprising the East Breaks Slides’bassociated with the late Pleistocene Colorado River and shelf-margin delta (Fig, l). Published seafloor maps’s”gare insufficiently detailed to show these channels, but they are apparent on recent NOAA multibeam data (Fig. 4). Paleogeography of the eastern Texas continental shelf and slope during the last glacio-eustatic lowering of sea level was compiled from several sources23’4”’O° “’2’’3”4 and summarized in Fig. 1, From the Ingleside strandline of the preceding highstand”, the shoreline migrated 75-200 miles southward to the shelf edge. The Trinity, Neches, Sabine, and Calcasieu (not shown) Rivers became entrenched and joined on the inner shelf to form a single river which extended southward toward the . 12 Neu tie shelf m~gin, this “greater Trinity” shelf margm . river was joined by the Brazes River’4. This combined river system ultimately built a deltaic complex at the shelf margin3.4”’0’’3”4. Suter and Berryhill’Omapped two discrete shelfmargin deltas (“A” and ‘B” in Fig. 1), whereas this study recognized a single, continuous shelf-mwgin delta complex whose maximum progradational extent occurs at the head of the Brazes-Tnnity fan system (Fig. 1). Prior to high-resolution seismic acquisition, gross features of the fan system Fig. 3) were mapped using conventional 2D seismic profiles (Fig. 5, 6). Basins I, 11,and IV are typical bowl-shaped salt-withdrawal basins, while Basin 111 is a graben. Basins I, II, and HI are eaeh filled to their bathymetric spill points with onlap-fiIl sequences. Each of these onlap-fill packages is incised by a surface channel system which bypassed sediment to the next basin(s) downslope. In contrast, Basin IV is only partly filled with an onlap-fill sequence and represents the ultimate terminus for the surface channel

Dealing with shallow-water flow in the deepwater Gulf of Mexico

Shallow-water flow (SWF) may occur while drilling shallow overpressured formations at deepwater sites. It is a high-profile problem in the Gulf of Mexico, although it does occur elsewhere and will likely be encountered in other deepwater regions (Figure 1).

ABSTRACT: Bonga Field, Deepwater Nigeria: Comparison of Near-Surface, Well-Calibrated Submarine Channels with Reservoir Channel Sands

Comparison of near-surface channels with channel reservoirs at Bonga, offshore Nigeria (OML118, 1000 m water depth), demonstrates some important lessons in using shallow analogs for reservoir prediction. Studies of shallow hazards and pipeline routing, integrating 3D and high-resolution 2D seismic with well logs, illustrate seafloor and nearsurface features including a large mud-draped canyon, mud volcanoes, and numerous pockmarks, which indicate fluid expulsion. Both the shallow overburden (Plio-Pleistocene) and the main reservoir intervals (Miocene) are characterized by deepwater channel geometries visible in seismic profiles, on map views, and within 3D volume views. Most near-surface channel features inhabit broad, relatively straight scours 1-3 km wide and 30-300 m deep, filled by composite packages of smaller, often sinuous channels 100500 m wide and 5-50 m deep. Although many of these near-surface channels display a chaotic to low-continuity, high-amplitude seismic facies character often associated with sandy fill, well logs through the near-surface section indicate these are mainly filled with mud. Channel geometries at reservoir level show different characteristics. The smaller-scale map-view geometries are consistent in size with the near-surface channels but the larger host scours are not as prevalent or obvious, and some thin, sheet-like sands are also present. A transition from lower slope to upper slope at the end of the Miocene probably accounts for this variation. A fundamental understanding of geologic setting and rock/fluid variation is critical before extrapolating seismic facies information from shallow analogues to deeper reservoirs.

Evolution and Seismic Expression of Mesozoic and Cenozoic Shelf Margins, Gulf of Mexico and Vicinity: ABSTRACT

Mapping and classification of modern and ancient shelf margins provide a basis for a concise post-rifting history of the Gulf of Mexico basin. Many hydrocarbon occurrences in the basin can be related to styles of shelf margins and their associated slope with implications for frontier exploration in deep-water facies. Seven basic types of shelf margins have been recognized. 1. Reef-dominated carbonate margins surrounded the deep basin during the Early and middle Cretaceous. This category can be subdivided into: (a) low-relief, short-lived (ca. 10 m.y.) margins in Louisiana and Texas (Stuart City and Sligo), and (b) high-relief (1 to 3 km; 3,280 to 9,843 ft), long-lived (ca. 40 m.y.) margins elsewhere (Florida, Campeche, and Blake escarpment, Golden Lane, El Abra), which are typically associated with fore-reef talus. 2. Sigmoidal progradational carbonate margins developed landward of drowned mid-Cretaceous margins of the Florida and Yucatan platforms during the Cenozoic. Large-scale gravity slides with rollover structures have occurred along the Yucatan slope contemporaneously with deposition of sigmoidal carbonate margins. 3. Carbonate ramps with little or no seismic expression characterized the Late Jurassic, when a deep marine basin was first established, an the Late Cretaceous, following drowning of mid-Cretaceous carbonate platforms. 4. Stable progradational clastic margins with well-developed, undeformed, large-scale clinoform stratification are relatively rare in the Gulf basin. Modern examples are limited to offshore Alabama and Veracruz. 5. Unstable progradational clastic margins result from gravity sliding of the continental slope, commonly associated with salt and shale diapirism, which obscures large-scale clinoform stratification. Growth faults with major expansion and rollover characterize the shelf margin; folds and/or thrust faults characterize the lower slope. This type of shelf margin has dominated the northwestern Gulf during the Cenozoic, and now extends from offshore Mississippi to offshore Veracruz. Numerous episodes of deltaic progradation to the shelf margin have been identified in Texas and Louisiana; these clastic influxes have prograded the shelf edge as much as 350 km (215 mi). 6. Tectonically active progradational clastic margins are present in Tabasco and eastern Veracruz. There the shelf margin has prograded up to 200 km (125 mi) since the early Miocene in an area of active compression and sinistral wrench faulting. Interaction of basement tectonics with diapirs and probably gravity sliding has created great structural complexity in thick Neogene deltaic sequences. 7. Tectonically active sediment-bypassing margins characterize the early Tertiary. The continental margins of Veracruz and Cuba underwent major compressional deformation and did not develop broad constructional shelves; instead, most sediment was probably bypassed to the deep basin. At the same time, a deep-water foreland basin, probably with a similar type of margin, extended into Chiapas and central Guatemala. End_of_Article - Last_Page 570------------

Cenozoic Shelf Margins, Northwestern Gulf of Mexico: ABSTRACT

Syndepositional gravity tectonics in the northwestern Gulf of Mexico basin have obscured the geometry of Tertiary shelf-slope stratification and thereby inhibited the recognition of relict shelf edges. However, examination of the modern shelf margin, formed primarily by deltaic deposition during the late Pleistocene lowstand of sea level, can lead to alternative criteria for recognizing Tertiary shelf margins. Late Pleistocene shelf-margin deltas, in contrast to inner-shelf deltas, are characterized by rapid subsidence and growth faulting, thick progradational cycles, and steep clinoform stratification. High subsidence rates result from deep-seated gravity sliding of the continental slope, which creates a strongly extensional regime along the shelf margin. Many downdip Te tiary formations are similarly characterized by large growth faults with high expansion ratios in deltaic sequences; hydraulic isolation of shallow-water sandstones by large fault offsets leads to overpressuring. These structurally-complex downdip trends, typically with geopressured gas reservoirs, represent the shelf-margin megafacies. Mapping of these shelf-margin trends provides a concise summary of the Cenozoic depositional and structural history of the basin. Major influxes of sand to the shelf margin correspond to episodes of rapid progradation and are interpreted as large shelf-margin deltas. Pre-Pleistocene shelf-margin deltas do not appear to be synchronous across the basin, and therefore are probably a function of sediment supply rather than sea-level fluctuations. The three largest such Tertiary delta complexes can be correlated with major tectonic episodes in likely source areas in western North America: (1) the late Paleocene (lower Wilcox) Rockdale delta system in east Texas coincides with the major pulse of Laramide uplift in the southern Rockies; (2) the mid-Oligocene (Frio) Norias delta system in sou h Texas coincides with extensive ash-flow volcanism in the Sierra Madre Oriental; and (3) the Neogene ancestral Mississippi delta system in Louisiana coincides with reactivation of the southern Rockies and regional uplift. End_of_Article - Last_Page 1440------------

Unstable Progradational Clastic Shelf Margins: ABSTRACT

Ancient shelf margins have generally been overlooked in some progradational clastic systems such as the northwestern Gulf of Mexico and the Niger delta. Apparently the contemporaneous structural deformation, particularly growth faulting, obscures depositional dips and foreset-topset geometry, making recognition of shelf breaks from these criteria virtually impossible. Nonetheless, their positions can be estimated from their association with characteristic micro-faunal assemblages, with initiation of growth faulting, with facies changes, and with geopressure. Rapid subsidence of progradational shelf margins results primarily from three processes: isotatic depression of the basement due to sedimentary loading, extensional thinning of the sedimentary wedge due to gravity tectonics, and compaction. Instability of the continental slope causes substantial basinward mass transport by deep-seated gravity sliding. This is manifested as down-to-basin listric growth faults originating at the outer shelf and upper slope (extensional regime), and shale and salt ridges and domes originating at the lower slope (compressional regime). The rapidly subsiding shelf margin acts as a major sediment trap, leading to accumulation of thousands of feet of shallow-water sediments, including deltaic sandstones, along a growth-faulted trend that may be hundreds of m les long. Shelf-margin deltas differ substantially from shallow-shelf deltas in that they show thicker and better differentiated progradational units and steeper clinoforms. Sand geometry of shelf-margin deltas is influenced by two competing factors: absence of a broad shelf to attenuate wave energy, thus favoring wave dominance, and high sand continuity, versus rapid subsidence, which prevent lateral reworking and thus favor river dominance and low sand continuity. Rapid downfaulting of shelf-margin deltaic sandstones against dewatering slope shales leads to the accumulation of excess fluid pressure in deep fault-bounded reservoirs. Mapping of geopressure trends can therefore provide a generalized picture of shelf-margin progradation in Cenozoic basins. End_of_Article - Last_Page 1008------------

Lower Wilcox Shelf Edge in Texas--Relation to Growth Faulting and Geothermal Reservoir Quality: ABSTRACT

Most geopressured sandstone reservoirs in the lower Wilcox (Eocene) of Texas occur along a narrow trend associated with the ancient shelf margin. Traps for geopressured fluids were created by early, rapid growth of down-to-the-basin faults as part of a large-scale instability of the continental slope. Basinward translation and rotation of upper-slope fault blocks over a decollement zone of geopressured shale (south Texas) or salt (east Texas) initiated fault movement near the shelf break. After the shelf edge had prograded farther basinward, continued movement of some of these faults at a much reduced rate created normally-pressured traps in post-lower Wilcox formations. In east Texas, the shelf-edge structural style has been overprinted by growth of salt dome. Faults tha originate near the restricts the volume of potential geothermal reservoirs. Lower Wilcox deposition was dominated by the Rockdale delta system in east Texas, similar in scale to delta system of the Quaternary Mississippi depocenter. Sand distribution reached its maximum extent when deltas prograded to the shelf edge. The thickest geopressured sands occur in De Witt County at the southern edge of the Rockdale system. Maximum permeabilities occur in distributary sandstones; these are laterally continuous with extensive delta-front sandstones of lower permeability. A proposed test-well site for the Cuero geothermal prospect in De Witt County has been located to intersect the greatest total thickness of distributary sandstones. End_of_Article - Last_Page 804------------

Distribution and Significance of Coarse Biogenic and Clastic Deposits on Texas Inner Shelf: ABSTRACT

Sediments of the Texas inner shelf are fine grained; coarse clasts (> 0.5 mm) are uncommon (< 1%) over much of the area. Higher concentrations of coarse material, however, occur in discrete areas which apparently represent positions of former deltas. Coarsest constituents are predominantly whole shells and shell fragments with subordinate amounts of lithic clasts. The calcareous skeletal debris represents a mixture of extant shelf fauna and relict brackish-water mollusks including Ragina sp. and Crassostrea virginica. Rounded sandstone and mudstone clasts up to 7 cm long and caliche nodules are common in some areas. Maps showing (1) coarse-fraction percent, (2) distribution of brackish-water mollusks, and (3) rock fragments show similar trends outlining ancestral Ri Grande, Brazos-Colorado, and Trinity deltas. A patchy, arcuate trend between Pass Cavallo and Aransas Pass is enigmatic. Criteria used to determine relative ages of shell debris for each of the four trends are degree of abrasion, fragmentation, etching, boring, and discoloration. Possible explanations for concentration of coarse material include high productivity, low rates of terrigenous clastic sedimentation, selective deposition by modern shelf processes, and reworking of locally shelly relict deposits exposed on the seafloor during the Holocene transgression. However, no single explanation adequately accounts for areal variations in coarse material. Reworking of delta-plain and estuarine deposits during and following sea-level rise is common to all areas which at present are also sites of insignificant coarse-sediment influx. Sabine-Bolivar trends are interpreted as transgressive lags derived from erosion of a late Pleistocene Trinity delta previously dissected by the Sabine River during Wisconsin glaciation. In contrast, Brazos-Colorado and Rio Grande tre ds are interpreted as possibly compound strandline features associated with subsidence, erosion, and retreat of Holocene deltas. Upwelling of nutrient-rich shelf waters and freshwater inflow also may have increased productivity of shelf benthos near the Rio Grande delta. End_of_Article - Last_Page 1608------------