Mark E. Odegard

Seismic studies on the Ontong Java Plateau, 1970☆

Abstract Marine seismic studies were conducted over the Ontong Java Plateau in the south-western Pacific Ocean during 1970 using two ships. Seismic refraction surveys revealed that crustal thickness of the plateau ranges from 35 to 42 km. A high-velocity basal crustal layer was detected in the northern part of the plateau but not in the southern part. Seismic reflection profiling showed the sediments to be about 1.5 km thick. In the sedimentary column were numerous intrusions with very little magnetic expression. These intrusions are probably not of igneous origin. There is a large discrepancy between observed gravity values and theoretical values calculated from the crustal structure.

Compressional faulting of the oceanic crust prior to subduction in the Peru-Chile Trench

An oceanic crustal section has been derived for the Nazca plate and across the Peru-Chile Trench at lat 12°S. Although based on unreversed seismic refraction data obtained by the Airgun-Sonobuoy-Precision-Echo-Recorder (ASPER) technique, 12 crustal determinations along the profile provide sufficient data density to permit good correlation of velocity structure between the stations. A thin, high-velocity oceanic crust characterizes the region. A major apparent offset in the crustal layers is interpreted as a low-angle thrust fault dipping east at an average of 6° from the ocean floor 300 km west of the trench axis. This feature, together with other indications of thrust faulting and crustal foreshortening, is taken as evidence that the upper lithosphere, at least to the depth of the Moho, is undergoing compression prior to subduction in the trench.

Satellite-derived gravity having an impact on marine exploration

Converting sea-surface height variations, derived from satellite altimetry, to free air gravity is not new. In the early 1980s William Haxby (Lamont Doherty Geological Observatory) produced the first global marine gravity map from SeaSat satellite altimeter data using interorbital track spacing of about 180 km. Haxbys map had a significant impact on plate tectonic theory because marine free-air gravity data were able for the first time to uniformly image the tectonic fabric of the earths oceanic crust. Since that time, much effort has been applied to improving satellite-derived gravity resolution. A major advance occurred in 1995, when the altimeter data from Geodetic Missions (GM) of GeoSat and ERS-1 satellites were released. Table 1 gives details of these satellites.

Nonlinear least-squares inversion of traveltime data for a linear velocity-depth relationship

Nonlinear least‐squares inversion of traveltime data is applied to the problem of determining thicknesses, velocities, and velocity gradients in laterally homogeneous, horizontally layered structures. The parametric forms of the traveltime equations are used for the calculations. Results of inversions on randomly inaccurate synthetic data show that the method will not determine the gradient consistently when using reflection traveltimes. Good results are obtained, however, when using traveltimes of energy refracted in a layer by the velocity gradient. Thicknesses and average velocity in the case of reflections, or velocity at the top of the layer in the case of refractions, are also determined. Partial derivatives determined during the course of the least‐squares inversion can be used to place limits on errors in the determined parameters.

Constraints for plate reconstruction using gravity data—implications for source and reservoir distribution in Brazilian and West African margin basins

Abstract Evaluation of tertiary sequences of West Africa and Brazil typically requires large budgets and staffs to identify drilling targets. Since these tertiary sedimentary systems extend for hundreds of kilometers (Proc. Petrol. Geol. Deepwater Depositional Syst. (2001) 2; Marine Petrol. Geol. (1990) 94–122), well beyond the limits of individual 3D and many 2D seismic surveys, additional information is necessary to interpret the most favourable locations for detailed exploration. Potential field data are powerful but often underutilized assets in building an exploration framework for reducing costs and interpretation risks. Although drilling locations are normally based on 3D seismic interpretations, reservoir distribution is controlled by features mappable with other methods such as potential fields. Other factors crucial to petroleum maturation, migration and trap formation relate to deep-seated controls that are well imaged (and sometimes exclusively imaged) on potential field data. Our observations from potential field data help identify reservoir distribution; source pod locations, source maturity, possible migration pathways; and potential traps. This paper illustrates the power of combining both regional and basinal scale interpretations based primarily on gravity analyses with extensive knowledge of the underlying geology. We review data sets and methods employed and describe results of a regional overview based on special plate-tectonic reconstructions of the South Atlantic. Examples from basin-scale work offshore West Africa and Brazil are used to illustrate factors important to hydrocarbon exploration. We conclude with a look ahead to improvements in methodology and application.

Advances in gravity survey resolution

Major technological advances have been made in the last few years in gravity resolution for many acquisition systems currently available for exploration. These advances have resulted from better instrumentation, better use of DGPS, and better processing methods. This in turn led to a renaissance in the use of gravity in modern multidisciplined, cost-effective oil and mineral exploration. The aim of this article is to show how gravity resolution has improved with time rather than how improved resolution is being used to investigate and map subsurface density structures.

Petroleum systems asymmetry across the South Atlantic Equatorial Margins

Abstract We describe an examination of two lines of evidence, tectono-structural evolution and hydrocarbon geochemistry, of asymmetric opening of the Atlantic Equatorial Margin. Our structural mapping used compilations of geophysical data and a review of both published literature and oil company public presentations. Geochemically, we accessed regional non-exclusive oil studies of the conjugate margins of Africa and South America, plus considerable published material. A group of non-exclusive oils was refined to 286, which clustered into five families, all represented along the NE Brazil margin but only one along the West African Transform (WAT) margin. Multiple lacustrine-sourced oils were seen around the South Atlantic, including NE Brazil, but a rich, oil-prone lacustrine source was not indicated offshore Ivory Coast and Ghana. Despite minor evidence of mixed source, possibly lacustrine stringers within an alluvial to marine setting, the predominant source is marine Cretaceous (Cenomanian–Turonian and possibly Albian). We find that opening asymmetry (a) biased the location of lacustrine (Early to mid-Cretaceous prerift to early synrift) source rocks to the NE Brazil margin and (b) locally narrowed the width of the optimal marine (Mid-Late Cretaceous postrift) WAT Margin source kitchens. Burial of the latter has aggravated the risk of late charge from light (condensate and gas) hydrocarbons.

Depth to basement using spectral inversion of magnetic and gravity data: Application to Northwest Africa and Brazil

Depth to basement is important in our exploration efforts, particularly for the determination of areas where there may be mature hydrocarbons. The use of spectral methods for the inversion of magnetic and gravity data to determine depth to basement has been known for some time. It has seen limited use in practical applications. We have developed a software package that implements this theory. This software application is fast and easy to use, so that large areas can be covered quickly. The method also averages over an area so that, if noise is a factor, the results will give a more accurate result than other methods that are commonly used. The drawback is that the resolution is limited to about one-half the size of the averaging window. Here we describe the method, discuss its application, and apply it to real data in the Brazil and Northwest Africa areas.

Sediment thickness, basement structure and tectonics from inversion and modeling over northeastern South America

This paper presents the techniques and results of a depth to basement inversion and modeling effort over the Guyana, Suriname, and French Guiana area of northeastern South America. A new development in the application of spectral inversion of both gravity and magnetic data has allowed us to extend the inversion over the entire study. Integrating the results of the spectral inversions with Euler deconvolution and 2-D modeling results in an accurate and informative description of the tectonics and structure of the area. Results show that significant packages of sediments of exploration interest have been deposited over most of the offshore area. This indicates that the USGS (2000) estimate of over fifteen BBOE for the area is highly possible.