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Mapping Sand Fairways and Salt Structures Using Modern Techniques Applied to South Louisiana Gravity Data [Abstract]
South Louisiana is an ideal region for using gravity data as an aid in geologic interpretation. The area has an abundance of salt features that generally have a large density contrast with surrounding sediments, typically causing negative gravity anomalies over salt and positive gravity anomalies over the sediments around the salt. The anomalies are often masked by longer-wavelength effects in the observed gravity field, which need to be removed. Once the longer wavelengths are removed, either mathematically or geologically through modeling, the residual gravity fields can then be more readily interpreted for mapping and modeling purposes.
Onshore gravity data, obtained from Gravity Map Service, covers all of South Louisiana and portions of southeast Texas and southwest Mississippi. The Bouguer gravity anomaly field contains the sum of gravity effects derived from all geological sources within the sedimentary section, the crystalline basement, and from the large density contrast across the deep-seated lower crust/mantle interface (Moho). The residual field that remains after removal of the regional gravity component is that part of the field that contains the effects of sources that are of direct or indirect exploration interest. The objective of the regional/residual gravity field separation is to obtain gravity fields that are observed to correlate with known features of interest, both in the sedimentary section and basement, and to project information into areas where there is less data or geologic information.
We applied a series of frequency-domain filters that enhance certain aspects of the input Bouguer gravity data, while suppressing others. High-pass, low-pass, and band-pass filters can be custom-designed to emphasize or remove anomalies having energy concentrated in particular spectral bands. Such filters are useful for qualitative enhancement of gravity features related to bodies located within a specific depth range, or removal of spectral energy associated with a particular signal character. Filter maps were created that include various derivatives, high-pass filters, and horizontal gradient filters. High-pass and low-pass residual/regional filters, along with derivatives, were identified as the most useful for sediment fairway mapping in the study area.
Of geologic interest, the high-pass gravity anomaly map of onshore South Louisiana is similar in appearance and scale to modern day continental slope bathymetry within active salt basins. These active salt basins lie directly down dip from Plio-Pleistocene main deltaic depocenters near the modern shelf edge from offshore central Louisiana through offshore southeast Texas. By using the Plio-Pleistocene analogy, which identifies the similarity between the residual gravity anomaly map and the continental slope bathymetry, it is possible to construct meaningful slope sand fairway and paleogeographic maps.
In constructing slope sand fairway maps, it is useful to take away the amplitude information by assigning a single color such as blue for gravity minima (salt-prone, less dense areas) and yellow for gravity maxima (sediment-prone, more dense areas) in order to give the anomaly map a typical net sand isopach appearance. Next, we overlay the shelf margin and the areas of main deltaic depocenters. Lastly, slope sand distribution should be calibrated with existing well coverage on the paleo continental slope and folded into the interpretation, noting sand development within the fairways. Like the Plio-Pleistocene example, the active salt basins lie down dip from the active depocenters.
Conversely, minor depocenter areas lie laterally adjacent to the main depocenters. Down dip of the minor depocenter areas, the slope is often referred to as a by-pass zone. However, this zone frequently contains smaller amounts of sediment fill possessing some potential turbidite reservoirs. Often the minor depocenter slope has a much larger area of exploration because there is not a thick overlying section removing it from economically drillable depths like that of the main depocenter. Fillon and Phair (1999) discussed in detail this method of creating computer drawn sand maps from sparse slope sand data and using gravity anomaly maps to steer the sand values up and down the fairways. By using the low-pass or regional gravity anomaly map, it is possible to identify those slope sand fairways that lie on possibly high or perched blocks and may be thin and sand poor. These high blocks may also act as rafts, lifting section into economically drillable positions, whereas the expanded section containing the thickest sands may be too deep to drill economically.
Lineations interpreted from the horizontal derivative map are overlain onto the sand fairway map to give a hint of faulting trends. Northwest-Southeast trending lineaments in the data come from the deep fabric that developed during the opening of the many basins within the Northern Gulf of Mexico during the Mesozoic. These deep lineations controlled the distribution of early salt, acting as boundaries for the local petroleum systems, and ultimately control where the sand was deposited. Additionally, it is also useful to overlay field outlines, top-of-salt contours, geologic maps, or any other type of information that may seem useful onto the fairway maps.
Combining density data, seismically interpreted surfaces, and the information gleaned in the salt/sand fairway mapping, we advance to quantitative interpretation using gravity modeling to help refine and/or corroborate salt structures. The seismically interpreted surfaces are used to construct a 3-D earth model. Density information taken from well logs is used to create a density versus depth function used as the background sediment density, whereas the salt is generally given a constant density in the model. Comparisons are then made between the computed gravity effect of the starting seismic model and the observed gravity field. At this point, we identify where the initial seismic model agrees with the observed gravity and where modifications need to be made. Modifications in South Louisiana are typically done on the base of salt, as the top of salt is often well defined by well log and seismic data. Changes to the model are implemented by either forward or inverse modeling. Regional and residual separation is more often done geologically in 3-D modeling, rather than mathematically as was done in the qualitative mapping phase. By including deeper geologic surfaces in our model, such as the basement and Moho, we are able to "model" the longer wavelength gravity effects. Therefore, when we modify and revise salt structures in the sedimentary section we are working with a geologically-based residual field, where basement and deeper sources have been removed (by inclusion of these sources in the model). Results from this phase of the work, typically a modification to the base of salt, are compared directly to the seismic data for incorporation into depth migration.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
2Dominion Exploration and Production, New Orleans, LA
Copyright © 2003 by NOGS (The New Orleans Geological Society)