The tension between CO 2 dissolved at relatively high atmospheric pressure in the Hadean ocean, and H 2 generated as ocean water oxidized ferrous iron during convection in the oceanic crust, was resolved by the onset of life. We suggest that this chemosynthetic life emerged within hydrothermal mounds produced by alkaline solutions of moderate temperature in the relative safety of the deep ocean floor. Exothermic reaction between hydrothermal H 2 , HCOO − and CH 3 S − with CO 2 was catalyzed in inorganic membranes near the mound's surface by mackinawite (FeS) nanocrysts and “ready-made” clusters corresponding to the greigite (Fe 5 NiS 8 ) structure. Such clusters were precursors to the active centers (e.g., the C-cluster, Fe 4 NiS 5 ) of a metalloenzyme that today catalyzes acetate synthesis, viz., the bifunctional dehydrogenase enzyme (ACS/CODH). The water, and some of the acetate (H 3 C.COO − ), produced in this way were exhaled into the ocean together as fluid waste. Glycine ( + H 3 N.CH 2 .COO − ) and other amino acids, as well as tiny quantities of RNA, generated in the same milieu were trapped within tiny iron sulfide cavities. Energy from the acetate reaction, augmented by a proton gradient operating through the membrane, was spent polymerizing glycine and other amino acids into short peptides upon the phosphorylated mineral surface. In turn these peptides sequestered, and thereby protected, the catalytically and electrochemically active pyrophosphate and iron/nickel sulfide clusters, from dissolution or crystallization. Intervention of RNA as a polymerizing agent for amino acids also led to an adventitious, though crude, process of regulating metabolism—a process that was also to provide genetic information to offspring. The fluxes of energy and nutrient available in the hydrothermal mound—commensurate with the requirements of life—encouraged differentiation of the first microbes into two separate domains. At the bifurcation the Bacteria were to specialize in acetogenesis and the Archaea into methanogenesis. Representatives of both these domains left the mound by way of the ocean floor and crust to colonize the deep biosphere. Once life had emerged and evolved to the extent of being able to reduce nitrogen for use in peptides and nucleic acids, light could have been used directly as an energy source for biosynthesis. Certain bacteria may have been able to do this, where protected from hard UV by a thin coating of chemical sediment produced at a sub-aerial hot spring operating in an obducted and uplifted portion of the deep biosphere. Embedded in fresh manganiferous exhalites, early photosynthetic bacteria could further protect themselves from radiation by adsorbing manganese on the membrane. Organization of the manganese with calcium, within a membrane protein, happened to result in a CaMn 3 O 4 cluster. In Mn(IV) mode this structure could oxidize two molecules of water, evolve waste oxygen, and gain four electrons and four protons in the process to fix CO 2 for biosynthesis. All these biosynthetic pathways had probably evolved before 3.7 Ga, though the reduced nature of the planet prevented a buildup of free atmospheric oxygen until the early Proterozoic.

ABSTRACT Over the past decade in the Louisiana and Texas shelf, AVO technology has been primarily applied to confirming mature prospects before drilling and utilized in producing field development. We suggest that AVO may also be productively used in exploration programs, but only when properly calibrated to local geological and reservoir conditions. Calculation of dozens of AVO attributes on hundreds of miles of modern, high quality 2-D data in an area of already producing fields demonstrates that reliance on a single “quick-look” attribute ( e.g ., the product of normal incidence amplitude and gradient) can be particularly misleading. Important elements in an adequate AVO analysis include 1) sequence-stratigraphic context to establish systems tract boundaries, 2) calibration to local petro-physical variations, 3) development of appropriate sets of attributes designed to display reservoir and fluid content properties, and 4) forward modeling to quantify AVO effects for relevant acquisition, processing, and signal/noise (S/N) factors. Extension of AVO techniques to 3-D data provides better resolution, redundancy, and horizon mapping capabilities. Because offset can be considered an additional dimension, the power of 3-D volume interpretation software (auto-tracking and amplitude extraction) can be applied directly to AVO inline interpretations. Examples of AVO effects over producing reservoirs offshore Texas and Louisiana illustrate both the successes and pitfalls of AVO techniques. AVO can be an important adjunct to interpretation of amplitude anomalies, particularly when used in the context of local stratigraphic conditions.

ABSTRACT The widespread availability of high-quality 3D seismic surveys covering large areas has made the sequence stratigraphic analysis of lithofacies variations within these data sets both practical and of critical importance for exploration and production alike. We have found that volume seismic attribute analysis (including integrated reflection intensity, reflection heterogeneity, and reflection strength), calibrated to local conditions in wells which have penetrated the relevant strata, is a particularly valuable tool for mapping depositional facies patterns. The technique involves careful manual interpretation and editing of selected horizons along a dense grid of seismic lines and time slices. Horizons are chosen with full regard to the regional sequence stratigraphic context (from 2D seismic, regional 3D, and biostratigraphically documented well control). Unless the data set has extremely high S/N and is largely unfaulted, loop auto-tracking is not usually adequate for stratigraphic purposes. Frequent use of aids such as horizon flattening, various color displays designed to emphasize continuity, “zoom” and “unzoom,” composite sections tying wells, seismic correlation, and multi-frame windows helps to resolve questionable areas and establish internal consistency. Once a carefully edited, consistent, and geometrically correct interpretation has been achieved, meaningful volume attribute maps of systems tracts may be constructed. From the large number of attributes which can be calculated, usually a small number in a given sequence stratigraphic context will show the best correlation with known lithofacies and be most useful. These can be selected by extrapolation from known stratigraphic units in wells. We have found this procedure to be far more reliable than theoretical inference based on models of limited and generally unknown applicability. Facies attributions (alluvial, fluvial/coastal plain, lacustrine, near-shore marine, neritic, and slope environments) are supported by the attribute analysis. Sand-prone and source rock facies are identifiable, systems tract by systems tract. We believe volume attribute analysis has widespread general applicability and can be developed and utilized to solve a number of outstanding exploration and production problems.

Abstract The Dahomey Basin, located along the western portion of the Nigerian continental margin, is underlain by continental and transitional crust. It exhibits stable to moderately unstable progradation and is characterized by a well-developed shelf-slope break. Systems tracts are similar to those described for other stable progradational margins. In contrast, the Niger Delta Complex, located off the central and eastern portions of the Nigerian coast, exhibits highly unstable progradation. The shelf-slope break is commonly not well-developed. Instead, a ramp, characterized by relatively uniform dip, is present. Large growth faults cut the ramp and define the shelf margin. Systems tracts are similar to those in deep-water Neogene strata of the offshore Gulf of Mexico. The basin floor fan and slope fan depositional systems comprise the lower lowstand systems tract. Basin-floor fans in both areas are defined by a prominent reflection along their upper boundary. This reflection downlaps along the sequence boundary or abuts against the down thrown side of a growth fault surface. The slope fan contains channel-fill complexes characterized by chaotic bedding and less common large channel-fills, which exhibit concave-upward reflections. Most slope-f an strata exhibit discontinuous lo semi-continuous subparallel reflections. In the Dahomey Basin, slope fan deposits commonly pinch out at or near the shelf margin. In contrast, contemporaneous shallow-water facies are developed in the Niger Delta Complex. Deposition occurred from multiple point sources. Large amplitude anomalies in the upper lowstand (prograding wedge) suggest thick sheet sands are present in the Niger Delta Complex. These sands occur in both shallow-water (interpreted as delta front facies) and deep-water environments (interpreted as shingled turbidites). Water depths are constrained in areas in which well control is present by benthic foraminifera. Transgressive and highstand systems tracts are commonly very thin. Different exploration strategies are appropriate for each systems tract off both the western and central portions of the Nigerian Coast. The best plays off the central portion of the Nigerian coast are probably structural traps and combination structural and stratigraphic traps (this includes updip pinchouts), whereas hydrocarbons may occur in smaller pure structural to pure stratigraphic traps along the west coast of Nigeria.