Offshore Alabama Research Articles

Neogene Benthic Foraminiferal Biofacies, Paleobathymetry, and Paleoenvironments of a Gulf of Mexico Transect

AbstractWe document Neogene benthic foraminiferal biofacies changes on a depth transect of six Gulf of Mexico industry wells (sidewall and cutting samples) that is oblique to the coast and extends from Main Pass to Green Canyon (offshore Alabama to Louisiana, USA). Calcareous nannofossil and planktonic foraminiferal biostratigraphic control provides the framework to make interwell comparisons of the benthic foraminiferal biofacies; these comparisons provide the basis for paleobathymetric and paleoenvironmental interpretations and to identify useful benthic foraminiferal biostratigraphic markers in this region.Benthic foraminiferal faunas indicate that Neogene paleodepths were slightly shallower at the eastern wells and deepened towards the western wells. Calcareous benthic foraminiferal biofacies dominated by Uvigerina spp. indicate that paleoenvironments characterized by low-oxygen conditions and/or high productivity occurred periodically during deposition along the transect. This is supported by repeated occurrences of a distinctive assemblage of agglutinated foraminifera (known as the “Agua Salada Fauna”) that is typical of dysaerobic environments.Evidence of intensified low-oxygen/high-productivity environments are recorded in lowermost Middle Miocene sediments at some locations and are present at all wells in Upper Miocene deposits. In Upper Miocene-Pliocene sediments, oxygen levels appear to have been related to paleodepth, indicating that the development of lower oxygen conditions was the result of an expansion or migration of the oxygen minimum zone. Our results are consistent with a global cause for the expansion of the oxygen minumum zone during the Late Miocene and widespread increase in export production. Our study shows that despite problems in well cuttings (cavings, inconsistent sampling) and complications in regional salt and fault tectonics, well transects can provide coherent benthic foraminiferal biofacies patterns that reveal paleobathymetric and paleoenvironmental changes in the Gulf of Mexico.

Oxygen and carbon isotope analysis of the Mooreville Chalk and late Santonian-early Campanian sea level and sea surface temperature changes, northeastern Gulf of Mexico, U.S.A.

Oxygen and carbon isotope profiles for strata of the Mooreville Chalk (upper Santonian-lower Campanian) of the eastern Gulf Coastal Plain, U.S.A., show correlations with published curves for these isotopes. The δ 18O curve exhibits a strong similarity to the Exmouth Plateau δ 18O curve from ODP drilling sites offshore northwestern Australia, and the δ 13C curve can be correlated with the δ 13C curve from the English Chalk Trunch section. A thermal maximum probably occurred in the northeastern Gulf of Mexico during the latest Santonian (∼83.8 Ma), as evidenced by the minimum δ 18O values in the lower Mooreville beds. A δ 13C positive excursion occurs at the same stratigraphic level, which has been recognized as the “Santonian/Campanian boundary event” worldwide. After this event, ocean surface water temperature decreased throughout the early Campanian. This carbon isotope excursion is followed by a plateau in δ 13C values with a peak value occurring in a condensed section (∼80 Ma), which has been correlated to a downlap or maximum flooding surface on seismic data from offshore Alabama. The section characterized by increasing δ 13C values corresponds to a marine transgression. The interval characterized by decreasing δ 13C values corresponds to regression and progradation. The maximum flooding event occurred ∼0.8 Ma later than the thermal maximum event. The Mooreville chalk/marl cycles are most likely a product of fluctuations in siliciclastic sediment influx into the northeastern Gulf of Mexico modulated by the precession band of the Earth's orbital cycles. Higher carbon isotope values occur in the marl beds indicating that these beds were formed in a more anoxic/dysoxic environment characterized by higher clay, silt input and higher organic carbon accumulation.

Sequence stratigraphy and orbital cyclostratigraphy of the Mooreville Chalk (Santonian–Campanian), northeastern Gulf of Mexico area, USA

Cycles from the depositional sequence level to decimeter-scale rhythmic alteration of chalk and marl beds in the Mooreville Chalk (Santonian–Campanian) in the northeastern Gulf of Mexico area are studied employing an integrated biostratigraphic, cyclostratigraphic, and sequence stratigraphic approach. Two dominant frequencies, identified from the natural gamma-ray log from the Alabama Power Selma Site Test well, site 3 hole 3, are attributed to two of the Milankovitch cycles, the long eccentricity (413 ka) and eccentricity (100 ka) cycles. Parasequences, the building blocks of systems tracts, appear to be modulated by long eccentricity cycles. Individual eccentricity cycles were counted to establish a high-resolution orbital chronology, which agrees with established biostratigraphic chronology. A high-resolution record of temporal changes in sedimentation rate is established on the basis of cyclostratigraphic chronology. Two condensed sections are identified. The lower condensed section is found to correspond to a downlap surface recognized in seismic data in the offshore Alabama and Mississippi area. This condensed section and downlap surface are interpreted to represent the maximum flooding event of a third-order depositional sequence. The maximum flooding event is also represented by a marine flooding surface in eastern Mississippi and by the Mooreville tongue in eastern Alabama. The upper condensed section is found to correspond to a faunal abundance peak recognized in data from outcrop studies, and is interpreted to represent the maximum bathometric surface that was achieved as a result of a condition of rising sea level in association with low sediment input during the deposition of the early highstand systems tract.

Physical and Biological Effects of Sand Mining Offshore Alabama, U.S.A.

Abstract Physical processes and biological data were collected and analyzed at five sand resource areas offshore Alabama to address environmental concerns raised by potential sand dredging for beach replenishment. Nearshore wave and sediment transport patterns were modeled for existing and post-dredging conditions, with borrow site sand volumes ranging from 1.7 to 8.4 × 106 m3. Wave transformation modeling indicated that minor changes will occur to wave fields under typical seasonal conditions and sand extraction scenarios. Localized seafloor changes at borrow sites are expected to result in negligible impacts to the prevailing wave climate at the coast. For all potential sand excavation alternatives at borrow sites offshore Alabama, maximum variation in annual littoral transport between existing conditions and post-dredging configurations was approximately 8 to 10%. In general, increases or decreases in longshore transport rates associated with sand mining at each resource area amounted to about 1 to 2% ...

High temperature quartz cementation and the timing of hydrocarbon accumulation in the Jurassic Norphlet sandstone, offshore Gulf of Mexico, USA

Abstract Jurassic Norphlet Formation sandstone reservoirs in Mobile Bay (offshore Alabama, USA) produce gas from great depths (>6.4 km) and elevated temperatures (>200 °C). Quartz cement is concentrated at the top of these aeolian reservoirs forming a low porosity ‘tight-zone’ of widely variable thickness (3–58 m) above a more porous reservoir section. The extent of the tight-zone is independent of depositional facies and its thickness strongly influences well performance. Intergranular porosity in the Norphlet has been preserved by inhibition of quartz cementation due to the occurrence of robust grain-coating chlorite. Quantitative petrographic data reveal that chlorite grain-coat coverage is less in the tight-zone sands (mean = 92%) than in the reservoir sands (mean = 99%). Burial history and quartz precipitation kinetics modelling indicate that this seemingly minor difference in the completeness of grain coatings is sufficient to produce the observed differences in cementation and porosity. Quartz cementation to form the tight-zone took place under conditions of deep burial and high temperature. It followed in time the onset of pressure solution, emplacement of liquid hydrocarbons, and the precipitation of a solid hydrocarbon film (pyrobitumen) on the walls of the intergranular pores. Fluid inclusion microthermometry data indicate that volumetrically significant quartz cement precipitated at temperatures of 150 °C or greater from highly saline aqueous fluids. Hydrocarbon-bearing inclusions are notably absent in quartz cement of the tight-zone, implying that the pore fluids were predominantly brine during precipitation. Oil and gas associated with pyrobitumen evidently escaped from Norphlet traps prior to tight-zone cementation. Gas presently found in Norphlet reservoirs of Mobile Bay represents a relatively recent accumulation and is not the product of in situ thermal cracking of oil.

ABSTRACT: Sequence Stratigraphy and Seismic Stratigraphy of the Lower Cretaceous Section in the Northeastern Gulf of Mexico

ABSTRACT The Lower Cretaceous (LK) section in the offshore Alabama and Mississippi area has been a petroleum target since the 1970's in the Main Pass and the Viosca Knoll areas. To date, no comprehensive stratigraphic analysis of this section in these locations has been published. This work focuses on the characterization of the sequence stratigraphy and seismic stratigraphy of the LK strata offshore Alabama and Mississippi. Approximately 3,500 kilometers of 2D multi-channel seismic reflection data are interpreted and integrated with gamma ray, lithologic well log descriptions, and well log lithostratigraphic picks from more than 50 wells, and with 527 meters of core from wells in the study area and surrounding areas. Seven check-shot surveys are used to integrate the seismic and well log data, and software for PC is used to integrate and interpret the various data sets. In the study area, the LK section is composed mostly of carbonates with minor siliciclastic beds that were deposited in inner-middle shelf, shelf margin, slope, and basin environments. Diagnostic well log signature patterns were associated with different paleoenvironments. Associations of seismic facies were utilized to recognize the various depositional settings. Eight major third-order depositional sequences were recognized. This section overlies a Lower Cretaceous (Valanginian) unconformity associated with a significant hiatus and is overlain by an Upper Cretaceous (mid-Cenomanian) unconformity associated with a deposition hiatus of more than 10 million years in the Main Pass 253#6 well. Highstand systems tract and lowstand systems tract deposits are generally the only strata seismically recognizable. Transgressive systems tract deposits consist of a single seismic cycle or are not resolvable. The shelf margin, characterized by discontinuous skeletal reefs in the carbonate sequences, shifted basinward during the Early Cretaceous. The shelf edge experienced greater subsidence with respect to the back-reef and middle shelf areas. Tectonic activity, probably associated with salt movement, was restricted to the shelf margin and slope settings.

Abstract: The Mobile Area salt basin: Early evolution of salt structures and attendant hydrocarbon trends in offshore Alabama

Abstract: The Mobile Area salt basin: Early evolution of salt structures and attendant hydrocarbon trends in offshore Alabama

Depositional and Diagenetic History and Petroleum Geology of the Jurassic Norphlet Formation of the Alabama Coastal Waters Area and Adjacent Federal Waters Area

The discovery of deep (20,000 ft) gas reservoirs in eolian sandstone of the Upper Jurassic Norphlet Formation in Mobile Bay and offshore Alabama in the late 1970s represents one of the most significant hydrocarbon discoveries in the nation during the past several decades. Estimated original proved gas from Norphlet reservoirs in the Alabama coastal waters and adjacent federal waters is 7.462 trillion ft3 (Tcf) (75% recovery factor). Fifteen fields have been established in the offshore Alabama area. Norphlet sediment was deposited in an arid environment in alluvial fans, alluvial plains, and wadis in updip areas. In downdip areas, the Norphlet was deposited in a broad desert plain, with erg development in some areas. Marine transgression, near the end of Norphlet deposition, resulted in reworking of the upper part of the Norphlet Formation. Norphlet reservoir sandstone is arkose and subarkose, consisting of a simple assemblage of three minerals, quartz, albite, and K-feldspar. The present framework grain assemblage of the Norphlet is dominantly diagenetic, owing to albitization and dissolution of feldspar. Despite the simple framework composition, the diagenetic character of the Norphlet is complex. Important authigenic minerals include carbon ate phases (calcite, dolomite, Fe-dolomite, and breunnerite), feldspar (albite and K-feldspar), evaporite minerals (anhydrite and halite), clay minerals (illite and chlorite), quartz, and pyrobitumen. The abundance and distribution of these miner als varies significantly between onshore and offshore regions of Norphlet production. The lack of sufficient internal sources of components for authigenic minerals, combined with unusual chemical compositions of chlorite (Mg-rich), breunnerite, and some minor authigenic minerals, suggests that Louann-derived fluids influenced Norphlet diagenesis. In offshore Alabama reservoirs, porosity is dominantly modified primary poros ity. Preservation of porosity in deep Norphlet reservoirs is due to a combination of factors, including a lack of sources of cement components and lack of pervasive early cement, so that fluid-flow pathways remained open during burial. Below the dominantly quartz-cemented tight zone near the top of the Norphlet, pyrobitumen is a major contributor to reduction in reservoir quality in offshore Alabama. The highest reservoir quality occurs in those wells where the present gas water contact is below the paleohydrocarbon water contact. This zone of highest reservoir quality is between the lowermost occurrence of pyrobitumen and the present gas water contact.

Seismic Expression and Depositional Setting of the Late Oligocene-Early Miocene Carbonate Unit, Offshore Alabama : ABSTRACT

Seismic Expression and Depositional Setting of the Late Oligocene-Early Miocene Carbonate Unit, Offshore Alabama : ABSTRACT

Authigenic Muscovite and Stylolitization Timing, Jurassic Norphlet Formation, Offshore Alabama : ABSTRACT

Authigenic Muscovite and Stylolitization Timing, Jurassic Norphlet Formation, Offshore Alabama : ABSTRACT

Upper Miocene Dauphin Natural Gas Sands in Offshore Alabama : ABSTRACT

Upper Miocene Dauphin Natural Gas Sands in Offshore Alabama : ABSTRACT

New Insights into the Post-Middle Jurassic Subsidence and Depositional History of Offshore Alabama, Northeastern Gulf of Mexico, based on Backstripping Analysis : ABSTRACT

New Insights into the Post-Middle Jurassic Subsidence and Depositional History of Offshore Alabama, Northeastern Gulf of Mexico, based on Backstripping Analysis : ABSTRACT

Authigenic Muscovite and Stylolitization Timing, Jurassic Norphlet Formation, Offshore Alabama: ABSTRACT

Authigenic Muscovite and Stylolitization Timing, Jurassic Norphlet Formation, Offshore Alabama: ABSTRACT

Timing of hydrocarbon generation and accumulation in fault-bounded compartments in the Norphlet Formation, offshore Alabama

Analysis of the variation in depth of the gas-water contacts in the Bon Secour Bay and Lower Mobile Bay-Mary Ann Fields of offshore Mobile Bay, Alabama, together with fault seal analysis, implies that some of the pre-Cretaceous faults in these fields must be sealing. It is also likely that the large, post-Cretaceous Lower Mobile Bay Fault is dip-leaking. The gas currently reservoired in the Norphlet Formation has most likely been produced by thermal degradation of oils generated in the overlying Jurassic lower Smackover Formation. For a range of heat flow models appropriate for this area of the Gulf Coast, the onset of gas generation (defined as the time at which 10% of the maximum possible amount of gas has been generated) occurs at 104 −5 +10 Ma. The error bounds in this estimate are the 90% confidence limits, which were calculated by assuming that the times of onset of gas generation are log-normally distributed. The time of onset of gas generation is younger than the age of the sealing faults in the area, hence it is likely that gas generation occurred in situ in the Norphlet Formation, with re-migration occurring during the development of the post-Cretaceous Lower Mobile Bay Fault.

A Model for Quartz Cementation in the Norphlet Formation, Mobile Bay, Offshore Alabama: ABSTRACT

A Model for Quartz Cementation in the Norphlet Formation, Mobile Bay, Offshore Alabama: ABSTRACT

40Ar/39Ar Analyses of Authigenic Muscovite, Timing of Stylolitization, and Implications for Pressure Solution Mechanisms: Jurassic Norphlet Formation, Offshore Alabama

AbstractThree authigenic muscovite morphologies are associated with Norphlet Formation stylolitization observed in the Texaco Mobile Area Block 872 #1 well: l)large crystals of 1M muscovite, which grew in the stylolites with their c-axes parallel to the plane of maximum compressive stress; 2) fine-grained bundles of muscovite that occur as pore-fillings near stylolites; and 3) pods of fine-grained muscovite that exist within stylolite insoluble residue and that were precipitated as pore-filling muscovite before the host sandstone pressolved.The population of large crystals of 1M muscovite grew at 51 ± 9 Ma, pore-filling muscovites precipitated at 77 ± 22 Ma, and muscovite pods have ages of 86 ± 16 Ma, as indicated by 40Ar/39Ar laser fusion. Apparent ages indicate that stylolitization was coincident with the beginning of organic maturation Zone 5 and could be the product of reservoir fluid pressure fluctuations induced by gas leakage. The lower Smackover Formation source/seal rock, acting as a pressure relief valve, could have been compromised by microfractures occurring during hydrocarbon generation and expulsion. Decreases in reservoir fluid pressure would have acted upon the sandstone framework by increasing the effective overburden pressure, thus making the rock more susceptible to pressure solution.Stylolite frequency and quartz cement volume increase in the finer grained portion of the conventional core. Quartz cement volume correlates inversely to percent sandstone porosity. Apparent muscovite ages indicate that stylolitization occurred after hydrocarbon migration. Silica mobility was limited because pressure solution mineral products were precipitated from within grain films of irreducible water within the sandstone.Stylolitization of quartz grains accounts for a minimum of 34% of the quartz cement in the upper cored section of the Norphlet Formation and minimum of 17% of the quartz cement in the lower cored Norphlet Formation. Quartz cement volumes are based on stylolite insoluble residue thickness and weight measurements of pyrobitumen within and nearby the insoluble residue seams. Stylolitization of K-feldspar and precipitation of muscovite can release additional silica which may have precipitated as quartz cement.

Relationships of Seismic Amplitudes and Gas Content of the Miocene Amos Sand, Mobile Bay Area, Offshore Alabama

Mobil Oil Company has collected three-dimensional (3-D) seismic data over Mary Ann field in the Mobile Bay area, Alabama. Although the survey was designed and collected so as to image the deeper Norphlet Sands, amplitude anomalies in the image of the shallow Miocene Amos Sand are evident. Relationships are developed between the seismic amplitudes and net feet of gas in the Amos Sand at the few existing wells. These relationships are used to predict net feet of gas everywhere in the area of the seismic survey. The result is a contoured map of net feet of gas in the Miocene Amos Sand in Mary Ann field.

Diagenesis and the Convective Flow of Formation Waters: An Example from the Norphlet Formation (Jurassic), Mobile Bay, Offshore Alabama

Diagenesis and the Convective Flow of Formation Waters: An Example from the Norphlet Formation (Jurassic), Mobile Bay, Offshore Alabama

Depositional Texture-Dependent and Independent Diagenetic Control of Petrophysical Properties, Norphlet Sandstone, Onshore and Offshore Alabama

ABSTRACT Diagenetic factors influencing reservoir heterogeneity vary significantly throughout the region of Norphlet hydrocarbon production. Distribution of some diagenetic components in these eolian reservoirs is controlled by depositional texture, including preferential carbonate cementation of coarser-grained laminae, concentration of pyrobitumen in finer-grained laminae, localization of pressure solution seams, and concentration of cements in deposits of specific eolian subenvironments, such as interdunes. The distribution of these diagenetic components, which create local to widespread barriers and baffles to fluid flow, can be determined by depositional modeling. However, the distribution of other diagenetic components in Norphlet reservoirs, including quartz, clay minerals, and pyrobitumen, is independent of depositional texture and cannot be determined by similar modeling. Factors controlling the distribution of texture-independent diagenetic components include the availability of chemical constituents from external sources, past and present positions of hydrocarbon-water contacts, and the time available for diagenetic reactions to proceed. In onshore fields, such as Hatter's Pond field, the position of fluid contacts influences reservoir quality. Permeability is highest above the hydrocarbon-water contact where authigenic illite is less abundant. The opposite relationship occurs in offshore fields in Alabama coastal waters and Federal OCS, areas where sandstone below paleo or present hydrocarbon-water contacts has the highest reservoir quality. Up to four diagenetic zones may occur stratigraphically. In descending order they are: (1) the dominantly quartz-cemented tight zone at the top of the Norphlet; (2) an interval above paleo or present fluid contacts in which pyrobitumen grain coats reduce pore volume and constrict pore throats; (3) an interval between paleo and present fluid contacts which lacks pyrobitumen and has the highest reservoir quality; and (4) an interval similar to interval 3 that lies below the present gas-water contact. Delineation of controls on the distribution of these intervals is critical to evaluation of gas reserves in offshore areas. End_of_Record - Last_Page 520-------

3-D Seismic and Reservoir Modeling, Ram Prospect, Viosca Knoll Block 912, Offshore Gulf of Mexico

ABSTRACT Ram prospect is a large stratigraphic trap located in Viosca Knoll Blocks 912/956/957 offshore Alabama in 3500 water depth. Reservoirs are Pliocene and Miocene gas and oil-bearing deepwater sands deposited as fan complexes in an intraslope basin. The field has been proved up by a total of 12 well penetrations and is nearing the development stage. In an effort to predict reservoir performance and recovery efficiencies, 3D reservoir models were constructed using Exxon's in-house 3D modeling program (GEOSET). Reservoir simulation studies will be based upon these 3D geological models. 3D seismic data was used to map seismic attributes around the prospect and well control was used to calibrate the seismic attributes based on known reservoir characteristics and thereby derive a facies map for the entire field. Top/base structure, gross isopach, facies polygons, porosity and Vshale were input into GEOSET to define the overall reservoir container and fill. The paucity of well data was compensated for by using the 3D-seismic based facies as a guide to filling polygons and be creating from the real well data. These pseudowells aided in the correlation within and between polygons. The resulting 3D models (total porosity, effective porosity, Vsand) faithfully reflect the heterogeneity inferred from both 3D seismic data and well control and provide an excellent visualization of reservoir continuity - much better than models derived from well data alone. The models serve as a framework within which reservoir simulations can be performed and various sensitivities run. Additionally, the GEOSET porosity models can provide an alternative reservoir volume calculation. End_of_Record - Last_Page 507-------