Janet M. Combes

The Vicksburg Formation of Texas: Depositional Systems Distribution, Sequence Stratigraphy, and Petroleum Geology

The lower Oligocene Vicksburg Formation of the Gulf Coastal plain contains major petroleum reservoirs in the Rio Grande embayment and is an economically viable target in other areas of Texas. Knowledge of the distribution of Vicksburg depositional systems is essential to understanding sandstone concentrations and, therefore, is fundamental to effective exploration and production of the Vicksburg section. The depositional setting of the Vicksburg reservoirs, their position in a sequence stratigraphic framework, and the influence these factors have on the petroleum geology of the Vicksburg are the focus of this paper. Surface and subsurface geological and geophysical data provided the framework for an analysis of the depositional systems and the petroleum geology of the Vic sburg. The two primary Texas Vicksburg depocenters, the Rio Grande embayment and the Houston embayment, are separated by the San Marcos arch, a deep-rooted structural nose. Within the embayments, sand-rich deltaic complexes merged along strike with barrier/strand plains. Contemporaneous growth faulting controlled depositional patterns of shelf-edge deltas in the Rio Grande embayment, but had only a minor effect on the configuration of the shelfal deltas in the Houston embayment. Smaller wave-dominated shelf delta complexes interspersed with barrier/strand plains extended across the San Marcos arch. Updip of these sandy paralic depocenters, fluvial systems traversed mud-rich coastal plain units. Seaward of the paralic systems, sand and mud deposits prograded across and built up over the r lict Jackson shelf and shelf-margin shales. These depositional complexes are contained in the systems tracts of one eustatic (Exxon) sequence. Vicksburg production from each of the three structural regions of Texas is characterized by reservoirs from different systems tracts and of distinct, different depositional origins.

Considerations and pitfalls of high‐resolution seismic acquisition, processing, and data analysis: Gulf of Mexico, offshore Louisiana

Analysis of frequency content Shallow, high-resolution 2-D seismic data, with acquisition parameters designed to image coreand outcrop-scale stratigraphic features, were collected over a Pleistocene shelf edge delta offshore Louisiana. Deviation of the actual acquisition parameters from the acquisition design parameters led to complications in data processing. However, the data quality were still excellent. Frequencies up to 10001200 Hz were recovered from the subsurface to approximately 150 msec (112.5 m) below the seafloor, permitting resolution of beds as thin as 0.4 m. Frequencies of 400 Hz or greater are needed to resolve the detailed internal geometry of the shelf-edge delta, where theaverage reflection spacing between the major deltaic clinoforms is 2-3 msec (1 S-2.2 m). The deltaic clinoforms dip at angles up to 8°, and, at the CDP spacing of this survey, may be spatially aliased at frequencies above 850 Hz. Analyses of amplitude spectra (Figure 2) and filter panels (Figure 3) of the stacked seismic data show that frequencies up to 10001200 Hz were recovered from the subsurface to approximately 150 msec (112.5 m) below the seafloor. The vertical resolution of these data, estimated using Kallweit and Wood’s (1982) approximation that the practical limit of resolution is equal to 1/( 1.4 x maximum frequency), is 0.6 msec. This translates to a bed thickness of 0.4 m at a seismic velocity of 1500 m/s. The shelf-edge delta targeted by the high-resolution study has a detailed internal geometry, with an average reflection spacing between the major deltaic clinoforms of 2-3 msec (1 .S-2.2 m). A maximum frequency of at least 400 Hz is needed to resolve these features (Figure 3).

Forward modeling of Late Pleistocene shelf‐edge deltas offshore Louisiana

The University of South Carolina`s SEDPAK program has been used to construct a two-dimensional forward stratigraphic model of the Louisiana shelf edge from 168 ka to the present. With eustasy as the only time-varying parameter, the model effectively reproduces the geometries of major sequence stratigraphic surfaces, general lithofacies distributions, and paleobathymetric trends. Modeling also confirms that shelf margin delta sand bodies were deposited during sea level lowstands. A more accurate model could be obtained by slight variations in sediment supply and subsidence rates through time.