Deep Gulf Research Articles

Seismic Stratigraphy and Geologic History of Middle Jurassic Through Lower Cretaceous Rocks, Deep Eastern Gulf of Mexico

ABSTRACT A recent seismic stratigraphic investigation in the deep eastern Gulf of Mexico has resulted in the definition of eleven late Middle Jurassic through Lower Cretaceous sequences in the west Florida basin and on the adjacent west Florida slope. The age control is based on tentative correlations with nearby, better controlled study areas. This investigation has produced a general reconstruction of the regions' paleogeography and geologic history. Earliest Upper Jurassic (Oxfordian?) sediments were deposited in a gently sloping ramp-type setting, which evolved into more steeply dipping ramp-type settings during the Kimmeridgian(?) and Tithonian(?). These Upper Jurassic sequences were deposited as broad lobes overlying a (Callovian?) salt sequence. Differential subsidence across a Tectonic Hinge Zone (THZ) toward the end of the Jurassic possibly controlled the establishment of a shallow water rimmed carbonate platform margin at the present day Florida escarpment. Lower Cretaceous deposits are interpreted as aggradational back-reef platform sediments landward of the margin, while coeval deposits in the adjacent basin are interpreted as proximal slope talus, distal slope deposits and overlapping basin floor fans. The lobe geometries of the basin sediments and the presence of a low velocity channel fill on the adjacent Lower Cretaceous platform suggest the possible bypass of siliciclastic sediments across the platform during lowstands and their deposition as deep sea fans in the basin. This also suggests the presence of siliciclastic reservoir rocks in the basin, and could be analogous to the Permian Basin area in west Texas. During the Lower Cretaceous, the platform margin aggraded and steepened, which caused deep sea current systems to become intensified and focused along the margins' base. This intensification was responsible for altering depositional patterns and processes. For example, the earliest Cretaceous fan deposits were generally oriented perpendicular to the basin. With time, the lobes became oriented more parallel to the basin, culminating in the deposition of a large north-south prograding mound. Near the end of the Lower Cretaceous, extremely intense currents, probably due to sea level drops, preferentially eroded these depositional lobes and mounds, culminating in the Mid-Cretaceous sequence boundary (MCSB).

Submarine venting of brines in the deep Gulf of Mexico: Observations and geochemistry

Research Article| June 01, 1992 Submarine venting of brines in the deep Gulf of Mexico: Observations and geochemistry Paul Aharon; Paul Aharon 1Department of Geology & Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803 Search for other works by this author on: GSW Google Scholar Harry H. Roberts; Harry H. Roberts 2Coastal Studies Institute, Louisiana State University, Baton Rouge, Louisiana 70803 Search for other works by this author on: GSW Google Scholar Ron Snelling Ron Snelling 1Department of Geology & Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803 Search for other works by this author on: GSW Google Scholar Geology (1992) 20 (6): 483–486. https://doi.org/10.1130/0091-7613(1992)020<0483:SVOBIT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Paul Aharon, Harry H. Roberts, Ron Snelling; Submarine venting of brines in the deep Gulf of Mexico: Observations and geochemistry. Geology 1992;; 20 (6): 483–486. doi: https://doi.org/10.1130/0091-7613(1992)020<0483:SVOBIT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Brine-issuing vents have been observed at 1920 m water depth on top of Green Knoll, an isolated salt diapir rising seaward of the Sigsbee Escarpment in the northern Gulf of Mexico. The brines are seven times saltier than ambient sea water. Their venting, both by lateral flow and by diffusion upward through the underlying sediments, gives rise to drainage patterns similar to braided streams. Brine transport pathways are floored by unindurated red-orange deposits similar to the iron oxide deposits of the Red Sea brine pools. The relations between major and minor constituents (Na, K, Cl, SO4, Br, and Li) and the δ18O compositions indicate that brines are formed by congruent sea-water dissolution of the underlying Louann salt containing a mixture of halite, sylvite, and anhydrite. Net removal of Mg corresponding to a proportional increase in Ca suggests that dolomitization is active in the subsurface. Ferrous iron is extracted from the salt diapir and precipitated as ferric hydroxide on floors of drainage networks during the venting and mixing of the brines with the oxygen-rich ambient water. The Green Knoll brines, whose chemistry is uncompromised by severe diagenetic overprints, could serve as an end-member analogue for formation waters that have acquired complex chemical histories during their long migration in sedimentary basins. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural Restorations of Salt-Cored Anticlines, Mississippi Fan Foldbelt, Northern Deep Gulf of Mexico

Structural Restorations of Salt-Cored Anticlines, Mississippi Fan Foldbelt, Northern Deep Gulf of Mexico

Structural Geology and Evolution of the Mississippi Fan Fold Belt, Deep Gulf of Mexico (1)

The Mississippi Fan fold belt, located in the deep Gulf of Mexico beneath the upper and middle Mississippi Fan, is characterized mainly by basinward-verging anticlines and associated thrust faults. The fold belt extends approximately 300 km eastward and is approximately 50 km wide. Based on correlations with deep Gulf of Mexico seismic sequences (Challenger, Campeche, Lower Mexican Ridges, Middle Mexican Ridges, and Upper Mexican Ridges), the folded strata are interpreted to be Upper Jurassic through Miocene. Salt tongues or sheets in the lower slope have overridden and partly masked parts of the fold belt. The approximate minimum age of the folded strata suggests that the major deformation took place mainly during the middle to late Miocene. South of the fold belt, strata downlap onto a major middle Miocene surface (15.5 Ma) at the top of the Middle Mexican Ridges sequence in the deep basin. This downlap represents sediment derived locally from the growing anticlines, and marks the beginning of growth of the fold belt. Within the fold belt, the coeval reflector represents the oldest identifiable onlap surface for turbidites deposited in the developing synclinal basins. Onlap of younger (uppermost Miocene-lower Pleistocene) Mississippi Fan strata onto the anticlines indicates that the fold belt formed major topographic barriers after its formation and influenced the distribution of early fan sediment. The structurally highest folds have experienced at least several hundred meters of truncation by deep-marine erosion. In the northern and eastern part of the fold belt, the entire Challenger sequence (Upper Jurassic-Lower Cretaceous) has been deformed with 2000-3000 m of structural relief. This uplift involves a structural thickening of stratigraphic section in the core of the anticlines, suggesting that the structures are cored with mobilized salt. Some of the younger fan sequences locally decrease in thickness across the folds, indicating recurrent movement and growth of the salt in the core of the folds throughout the Pleistocene. In some places, salt has remobilized and completely penetrated the overlying fan sediments to the surface, thus forming diapirs. Basement rocks were not involved in the deformation. Thrust faults beneath the folds sole out into the lower part of the Mesozoic section (pro ably into Middle Jurassic salt). The folds and thrust faults are most likely the result of compressive stresses caused by updip sediment loading during the development of Miocene shallow-water depocenters to the north. These stresses were transmitted to the salt basin edge along a deep decollement surface or surfaces within, or at the top of, the Jurassic salt section. Thus, the original distribution of salt overlying transitional crust controlled the location of the fold belt, which formed where the salt pinches out and the decollement surface ends.

Lithospheric structure of the south-central United States

Recent seismic data in the Ouachita Mountains area and the Gulf of Mexico make it possible to construct a lithospheric-scale cross section (transect) from the midcontinent region to the gulf. The authors constructed a transect in the form of a gravity model, but it incorporates all available seismic, drill hole, and geologic data as constraints. The thrust sheets of the Ouachita orogenic belt appear as a thin veneer covering the southern part of the Arkoma basin and the preserved Paleozoic continental margin. Mesozoic rifting is evident in three areas: (1) southern Arkansas and northern Louisiana where extension was minor, (2) the vicinity of the Texas-Louisiana coastline where modification of the lithosphere and subsidence were considerable, and (3) the deep Gulf of Mexico where rifting was successful. A significant variation in the average density of the mantle, which could delineate the North American craton as a lithospheric feature, was detected near the Paleozoic continental margin.

Recognition and Significance of Sequences and Their Boundaries, Deep Gulf of Mexico

Recognition and Significance of Sequences and Their Boundaries, Deep Gulf of Mexico

Ecological History, Catastrophism, and Human Impact on the Mississippi/Alabama Continental Shelf and Associated Waters: A Review

The MississippilkIabama continental shelf and associated coastal waters together form a complex ecological system of interrelated parts. The: biological system of the area has become established during the period of sea level rise foHowhg the last continental glacial maxi about 18,000 years ago. Contemporary biologkal populations oftheinshore waters are subject toepisadiccatastrophic events caused by exceptional cold fronts, flooding, major storms, hypoxia,rsd tide outbreaks,andmajordroughts. Most of thesewenhare nothown toaffect the shelfpopulations directly, but inditect effects through had chain disruptions are likely. h a p Current intrusions and entrainment of deep Gulf waters could Tirectly impact the shelf species. Imposed upon these events are various human intrusions which have severely reduced the quality and quantity of inshore habitats. Increme in commercial and recreational fishing pressure in the inside w3kw and on the continental shelf during the past hvo decades has k e n accompanied by dramatic decline in popuia'tions of demersal and pelagic fish species. In order to be able 20 manage resources of the area successfully, lhwe is an urgent need to understand the natural functioning ofthe ensire complex ecological system.

Pleistocene-Holocene Lower Bathyal Benthic Foraminifera: A Pilot Study in Keathley Canyon, Northwestern Gulf of Mexico

Recent work on the shelf and upper slope have linked the distribution of benthic foraminifera to the presence of several Gulf of Mexico water masses. A pilot study consisting of three piston cores from lower bathyal depths (1,308 m, 1,543 m, 1,815 m) was undertaken to examine the distribution of benthic foraminifera across the Pleistocene-Holocene boundary and at several depths within the lower slope environment. The primary objective of this study was to test the hypothesis that abundance variations of benthic foraminifera can be used to refine the bathymetric zonation of deep Gulf of Mexico depositional environments based on their water mass associations. Preliminary results from this study support this hypothesis by showing a distinct variation in benthic foraminiferal abundances between the shallower cores (1,308 m, 1,543 m) and the deeper core (1,815 m). The cores from 1,308 m and 1,543 m contain a fauna that exhibits a moderate abundance (ca. 10-15%) of several species: Bolivina lowmania, Bulimina aculeata, Cassidulina subglobosa, Gyroidina soldanii and Oridorsalis spp., while the core from 1815 m contains a fauna strongly dominated (25-60%) by two species: Eponides turgidus and Nuttallides decorata. Abundance variations downcore or across the Pleistocene-Holocene boundary are subtle but present nonetheless. Themore » preliminary results from this study suggest that the distribution of deep Gulf of Mexico benthic foraminifera may be related to the distribution of water masses comprising the deep gulf and that further bathymetric refinement of the lower slope may be possible.« less

Post Mid-Cretaceous Seismic Stratigraphy and Geologic History, Deep Gulf of Mexico Basin

The deep Gulf of Mexico basin is underlain by up to 10 km of sedimentary rocks. This section is subdivided into two megasequences separated by a prominent, basinwide, mid-Cretaceous sequence boundary (MCSB), which marks a major turning point in the history of the Gulf basin. The post-MCSB section consists of the more distal portions of large siliciclastic wedges that partially fill the basin. A detailed seismic stratigraphic analysis has subdivided the post-MCSB, pre-Pleistocene section into 12 new sequences, which have been mapped throughout the entire deep basin using UTIG and all available industry seismic data. Correlation with two published Exxon seismic lines on the northeastern shelf provides tentative age control for the five youngest sequences. The correlation across the slope was made using a dense grid of UTIG and industry seismic data with minimum interference from salt structures. These ages were further confirmed by correlation with DSDP sites 90 and 91 in the western Gulf. This study suggests that the base of the Mississippi Fan is Upper miocene in age, which is older than the Plio-Pleistocene age proposed by previous studies. Isopach maps of each new sequence document the progressive filling of the deep basin and the shift of majormore » deep water depocenters from the west and northwest to the north. This corresponds to the shift of the deltaic depocenters along the northern Gulf margin. A tentative correlation of the deep Gulf sequences with the major northern Gulf coast depositional episodes is proposed.« less

Application of sedimentary carbohydrate residues in a study of organic facies and natural gas occurrences

Recent aquatic environments and resulting organic facies can be characterized by types and amounts of carbohydrate residues. Characteristics are based on source organisms, degree and type of degradation, and reactions with associated compounds in the mineral-kerogen-humus complex. Selected modern environments are typified by the following presently known carbohydrate suites: (1) deep sea, mid-Pacific, mid-Atlantic Oceans—glucose, galactose, furfurals, low total carbohydrates (TC); (2) deep gulf, Gulf of California—glu, gal, xylose, mannose, furfurals, moderate to high TC; (3) continental shelf, eastern North America—glu, xyl, gal, furfurals, high TC; (5) oligotrophic lake, Minnesota—furfurals, low TC; (6) eutrophic lake, Minnesota—glu, xyl, arabinose, gal, rhamnose, man, ribose, furfurals, glucuronic acid, high TC; (7) bog, Minnesota—glu, ara, xyl, gal, man, rib, very high TC. Polysaccharides are rare to absent in modern deep sea deposits but have been found in Lower Quaternary and younger deep gulf sediments. Cellulose, alpha- and beta-amylose and laminaran are common in shallow marine and lacustrine sediments. Methane, derived from both terrestrial and aquatic higher plant residues is high in yield in freshwater marshes and bogs and in eutrophic lake sediments, moderate in salt-water marshes and estuaries and relatively low in offshore marine sediments. Nitrogen and carbon dioxide are the commonest non-hydrocarbon gases. In many samples studied, xylans appear to predominate over other plant polysaccharide as methane sources. Carbohydrate residues in ancient rocks, based on examples from North America, show a tentative, but as yet poorly investigated, relationship to environmental organic facies and should prove to be useful in natural gas exploration.

Sequence Stratigraphy and Facies Variations within Neogene Turbidite Systems, Northern Deep Gulf of Mexico

The Neogene strata of the northern deep Gulf of Mexico are characterized by turbidite systems of different stages of evolution and depositional setting. These Neogene sequences are the basinal, downdip equivalent strata to the petroleum-producing intervals in the continental shelf and slope to the north and northwest. The middle to late Miocene Upper Mexican Ridges sequence (12.5-5.5 Ma) and Cinco de Mayo sequence (5.5-4.5 Ma) are interpreted to consist primarily of unchannelized to partially channelized basinal turbidite systems. Many small cut-and-fill features are present in the sequences; small channel-levee systems may be present only in a few places. The overlying Mississippi Fan (4.5 Ma to present) consists of 17 seismic sequences that have a much larger areal distribution than the two underlying Miocene sequences. Each fan sequence is characterized primarily by sediments deposited in channel, levee, and overbank settings. The differences among these turbidite sequences are caused by both the overall progradation of the continental margin and differences in the magnitude of sea-level fluctuations and volumes of sediment deposited in the deep Gulf of Mexico during the Neogene. The Upper Mexican Ridges and Cinco de Mayo sequences are interpreted to be the first coarse-grained turbidite systems deposited in this areamore » of the deep Gulf of Mexico, with the overlying Mississippi Fan being part of a continuum in deep-water depositional systems. The oldest sequences in the Mississippi Fan were relatively small and approximately the same size as the Upper Mexican Ridges and Cinco de Mayo sequences. The magnitude of sea-level fluctuations, which controlled the timing of deep-water sedimentation, was not as large during the middle to late Miocene as during the Plio-Pleistocene.« less

Age Determinations for New Deep Gulf of Mexico Basin Seismic Sequences

Age Determinations for New Deep Gulf of Mexico Basin Seismic Sequences

Ageostrophy and pressure work in the Gulf Stream at 73°W

During 1984, velocity, bottom pressure and thermocline depth data were obtained from deployments of tall current meter moorings, bottom pressure gauges and inverted echo sounders at five sites on the anticyclonic side of the Gulf Stream at 73° W. An additional number of inverted echo sounders, deployed in several nearby sites, allowed us to construct daily regional maps of the thermocline topography. Using this data base we examined the momentum balance and the pressure work in the eddy kinetic energy balance at 500 m above the seafloor and at a depth of 400 m. At 500 m off the bottom, the correlations between fluctuating geostrophic currents, inferred from pressure differences between sites, and suitably averaged observed currents provide observational confirmation of the geostrophic balance in the deep Gulf Stream, within our ability to measure it. At 400 m depth, the ageostrophic current was computed using velocity measurements, curvature estimates from the thermocline maps, and the momentum equation in natural coordinates. Comparison of observed and ageostrophic velocities showed occasional departures from geostrophy of O(30% to 60%) with duration from 2 to 5 days. These ageostrophic events were caused by tangential and centripetal accelerations, mostly during the coalescence of two cold‐core rings with the stream. The mean departure from geostrophy at 400 m during the deployment period was 7%. The mean effect of the pressure work, on the anticyclonic side of the stream at 400 m, was to decrease the eddy kinetic energy at a rate of −3×10−3 ergs cm−3 s−1; a value with magnitude as large as that of the barotropic instability term. The pressure work record is very episodic with peaks, during the ageostrophic events, close to 30 times larger than the mean estimate and with both positive and negative values. At 500 m off the bottom, the pressure work was found to be insignificantly different from zero.

Sequence Stratigraphy, Facies Geometries, and Depositional History of the Mississippi Fan, Gulf of Mexico (1)

The Mississippi Fan is a large, mud-dominated submarine fan over 4 km thick that was deposited in the deep Gulf of Mexico during the late Pliocene and Pleistocene. Analysis of 19000 km of multifold seismic data across the fan defined 17 seismic sequences, each characterized by a series of channel, levee, and associated overbank deposits, along with other mass transport deposits. At the base of nine sequences are a series of seismic facies consisting of mounded, hummocky, chaotic, and subparallel reflections, which constitute 10-20% of the sediments in the sequence. These facies are externally mounded in cross section and occur in two general regions of the fan. In the upper and middle fan, they occur below channels and are elongated in shape, mimicking the channel's distr bution. In the middle to lower fan, they have a fan-shaped distribution, increasing in width downfan. These facies are interpreted to have formed as disorganized slides, debris flows, and turbidites, and are informally called mass transport complexes. Overlying this basal interval and characteristic of all sequences are well-developed channel-levee systems, which constitute 80-90% of the fan's sediments. Channels consist of high-amplitude, subparallel reflections. Levee sediments have subparallel reflections that have moderate to high amplitudes at the base changing upward to low amplitude. The vertical change in amplitude may reflect a decrease in the grain size and bed thickness of the levee sediments. Overbank sediments consist of interbedded subparallel to hummocky and mounded reflections, suggesting both turbidites derived from the channel, as well as slides and debris flows derived from the slope. Pliocene-Pleistocene eustatic cycles are interpreted to have been the major factor controlling the timing and style of sedimentation in the fan. Mass transport complexes are interpreted to have formed during a lowering of sea level, and reflect sediments derived from retrogressive slumping during the formation of submarine canyons in the upper slope and outer shelf. Channel-levee systems were deposited when sea level was near its lowest position and sediment derived from deltas was transported into the deep basin via submarine canyons. During highstands in sea level, a thin layer of hemipelagic sediment was deposited on the fan surface. The Mississippi Fan serves as an exploration model for mud-dominated submarine fans and has four prospective reservoir facies: channel sands with linear trends, unchannelized sands beyond the downdip terminus of the channel (possible lobes), potentially sand-prone levees immediately adjacent to initial channels deposited in some sequences, and limited parts of mass transport complexes.

Organic Geochemistry of Sediments of the Deep Gulf of Mexico Basin: ABSTRACT

Organic Geochemistry of Sediments of the Deep Gulf of Mexico Basin: ABSTRACT

Sequence stratigraphy of the Mississippi fan (Plio-Pleistocene), Gulf of Mexico

The Mississippi Fan is a large, mud-dominated submarine fan over 4 km thick, deposited in the deep Gulf of Mexico during the late Pliocene and Pleistocene. Analysis of 19,000 km of multifold seismic data defined 17 seismic sequences, each characterized by channel, levee, and associated overbank deposits, as well as mass transport deposits. At the base of nine sequences are a series of seismic facies consisting of mounded, hummocky, chaotic, and subparallel reflections, which constitute 10–20% of the sediments in each the sequences. These facies are externally mounded and occur in two general regions of the fan: (1) in the upper and middle fan they are elongate in shape and mimic the channel's distribution; (2) in the middle fan to lower fan they are characterized by a fan-shaped distribution, increasing in width downfan. These facies are interpreted to have formed as disorganized slides, debris flows, and turbidites (informally called “mass transport complexes”).

Sedimentary framework of the southern Maine inner continental shelf: Influence of glaciation and sea-level change

Although the tidally influenced shoreline of Maine is longer than that of virtually any other state, almost no research on its geology has been published. In order to go some way towards remedying this, 1500 km of high-resolution seismic reflection data and 800 km of sidescan sonar imagery have been collected. On the basis of these data and observations made during ten submersible dives, more than 800 bottom samples were collected and evaluated for texture and composition. The understanding of the sedimentary framework of the southern Maine shelf and the processes that maintain it are summarized, and future research directions to evaluate the strategic mineral potential are indicated. In the past 14,000 years, the Maine shelf has experienced a deglaciation and two marine transgressions separated by a regression. The deglaciation was accompanied by the first transgression and deposited till interbedded with up to 40 m of glaciomarine sediment (the Presumpscot Formation) across the shelf. The first transgression culminated about 12,500 yrs B.P., and its landward limit is marked by large glaciomarine deltas 50–100 km landward of the present-day coast. Sea level fell until about 9500 yrs B.P., when shorelines were cut at about the 65 m depth and some large “lowstand deltas” were deposited. Sea level has risen since then and in the general absence of modern river sediment input marine processes have reworked the older sediment. Five shelf environments have been defined in terms of their surficial sediment and stratigraphy. Nearshore ramps are sandy regions extending to about 30 m deep offshore of sandy beaches. These may be reworked lowstand deltas, and possess the thickest bodies of sand in the region. Nearshore basins are mud-filled troughs seaward of coastal areas lacking significant river input. Slumping glaciomarine deposits provide most of the Holocene mud that floors these basins. Rocky zones are extensive areas of exposed rock most common in the 30–50 m depth range. These are areas of high carbonate productivity and provide shell fragments to adjacent areas. The outer basins are muddy regions that begin at the 65 m shoreline and extend beyond the 100 m isobath to the deep Gulf of Maine. Finally, shelf valleys are former stream courses that connect nearshore ramps and basins to the outer basins. These bedrock-framed valleys are filled with sand and mud and are apparently still active conduits for the offshore movement of sediment. The mineralogy of the shelf sediment is complex, and it is not yet clear whether glacial homogenization has obliterated distinctions between river drainage basins and the adjacent shelf. Studies are continuing into northern Maine and analysis of the heavy mineralogy of the shelf sediment utilizing a 2 m Humphrey Spiral to process large grab samples and cores has begun.

A tracer study of the deep Gulf Stream cyclonic recirculation

An advective-diffusive numerical model is used to interpret water sample data from beneath the thermocline in the region of the Gulf Stream and deep western boundary current (DWBC), west of the Grand Banks. The nearly homogenized distributions of salinity and oxygen suggest that substantial cyclonic recirculation of the deep Gulf Stream comes in close contact with DWBC, which is the source of the tracers. The exchange of properties between the two flows is studied in order to relate tracer distributions to particular characteristics of the Gulf Stream gyre. Using a corresponding diffusion box model, it is shown that spin-up and the onset of homogenization in the gyre is controlled by diffusion from the boundary, and that the level of tracer in the gyre can be insensitive to the strength of lateral diffusivity (the salt case), but is strongly sensitive to vertical mixing (the oxygen case). The presence of vertical mixing, however, does not inhibit the occurrence of homogenization. The lateral (κ) and vertical (ν) diffusivities of the Gulf Stream gyre can be deduced by applying the model results to the observed oxygen distribution, and this leads to estimates of κ ∼ 10 6 cm 2 s −1. The tight lateral extent of the recirculation, as suggested by the model results, agrees well with existing current meter data from the region.

Relationship of Pressure Regressions to Secondary Porosity Development: Exploration Model for Secondary Porosity Development in Deep Wilcox of South Texas: ABSTRACT

In the deep Wilcox, as well as other deep Gulf Coast Tertiary formations, secondary porosity development is very important to reservoir quality. four major Wilcox fields and several abandoned wells were studied with respect to shale pore-pressure gradients to show the relationship between secondary porosity and shale pore-pressure regression. Secondary porosity is the result of chemical leaching and alteration of framework grains and clays in a rock by organic acids, fresh water, and other fluids or gases reacting with the rock constituents. These chemicals, water, and gases are formed in shales adjoining the sandstone reservoir rock and migrate into the adjoining aquifer sandstone, which is then leached. This release of fluids from the shale forms a pore-pressure regression which can be plotted using electric logs. The presence or absence of a pore-pressure regression gives indications of secondary porosity development in the adjoining reservoir rocks. If a regression is documented, the risk of inadequate reservoir porosity of a prospect can be lowered.

Petroleum Potential of Mississippi Fan: Exploration Analogs to Flex Trend Reservoirs: ABSTRACT

The Mississippi fan is a large mud-dominated submarine fan over 4 km thick that was deposited in the deep Gulf of Mexico during the late Pliocene and Pleistocene. The fan contains deep-water sediments coeval to the Flex Trend turbidite reservoirs deposited in the continental slope to the north. The fan reflects turbidite deposition in an unconfined basin, in contrast to the confined intraslope basins. Analysis of 19,000 km of multifold seismic data across the fan defined 17 seismic sequences, each characterized by a series of channel, levee, and associated overbank deposits and other mass transport deposits. At the base of nine sequences is a series of seismic facies consisting of mounded, hummocky, chaotic, and subparallel reflections which constitute 10-20% of the sediments of each sequence. These facies are externally mounded in cross section and occur in two general regions of the fan. In the upper and middle fan, they occur below channels and are elongate in shape, mimicking the channel's distribution. In the middle to lower fan, they have a fan-shaped distribution, increasing in width downfan. These facies are interpreted to have formed as disorganized slides, debris flows, and turbidites and are informally called mass complexes. The Mississippi fan hasmore » four prospective reservoir intervals and serves as a possible exploration model for Flex trend reservoirs: channel sands with linear trends, unchannelized sands beyond the terminus of the channel downdip that are areally widespread, potentially sandprone levees immediately adjacent to initial channels deposited in some sequences, and limited portions of mass transport complexes. Large structural traps are present where the fan has been deformed by salt and the Mississippi fan foldbelt.« less