The Occurrence of the Greta Sandstone, Frio Formation (Late Oligocene), Texas Gulf Coastal Plain
The Occurrence of the Greta Sandstone, Frio Formation (Late Oligocene), Texas Gulf Coastal Plain

Author: Sean Christian Kincade

Publisher:

Published: 2018

Total Pages: 158

ISBN-13:

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The Greta sandstone is an informal lithostratigraphic unit of the upper Frio Formation (Oligocene) that occurs across the Texas Gulf Coastal Plain. The Greta sandstone has been known to geologists since it was first drilled in the 1930's, yet there has been very little research into this un-depleted and overlooked reservoir. This study provides insights into the lateral distribution and reservoir facies of the Greta sandstone. The unit represents an aggradational to transgressive barrier sandstone package grading up to mud-rich open marine inner shelf deposits. The interval is easily recognizable in well logs based on its strong SP and Gamma-ray responses. The depositional limits of the Greta sandstone within the Greta/Carancahua barrier/strandplain system has been mapped based on correlation of several thousand well logs throughout the south Texas Gulf Coast. This has aided in the understanding of the sequence stratigraphy of the Greta interval and the reservoir bodies that it contains. The Greta sand typically has 30% porosity and 280md permeability, produces heavy oil (~21API), and is commonly the uppermost producer in the Frio section. Cumulative and present day production data of the Greta sandstone has been collected to establish a database for oil production.

Depositional systems and tectonic/eustatic history of the Oligocene Vicksburg episode of the northern Gulf Coast
Depositional systems and tectonic/eustatic history of the Oligocene Vicksburg episode of the northern Gulf Coast

Author: Janet Marie Combes Coleman

Publisher:

Published: 1990

Total Pages: 1076

ISBN-13:

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Regional depositional systems analyses combining surface and subsurface geological and geophysical data provide the framework for a sequence stratigraphic study of the Lower Oligocene Vicksburg Formation of the Gulf Coastal Plain. The results describe the eustatic history of the Vicksburg stratigraphic unit. The two primary Texas depocenters, the Houston embayment and the Rio Grande embayment, were separated by a deep-rooted structural nose: the San Marcos arch. A barrier / strandplain intervened between the Louisiana deltaic depocenter and the Houston embayment. Within the embayments, deltaic complexes merged along strike with barrier / strandplains. Contemporaneous growth faulting controlled deltaic depositional patterns in the Rio Grande embayment and, to a lesser degree, in the Houston embayment. Smaller wave-dominated delta complexes interspersed with barrier / strandplains extended across the San Marcos arch. Updip of the paralic depocenters, fluvial systems traversed coastal plain units. Seaward of the paralic systems, sand and mud deposits prograded across and built up over the relict Jackson shelf and shelf margin. The contact between the Vicksburg Formation and the underlying Jackson Group marks the position of the Eocene - Oligocene boundary within the Gulf Coastal Plain section. On regional dip-oriented well-log cross sections there is a distinct, abrupt, seaward shift in the paralic facies at the Jackson - Vicksburg boundary; this contact corresponds to an Exxon-model Type 1 unconformity. The unconformity is related to the development of an Antarctic ice sheet in the earliest Oligocene. During middle Vicksburg time, a minor transgression (genetic stratigraphic sequence boundary) flooded the coastal plain. Overlying the progradational Vicksburg Formation, the lower Frio Formation accumulated in an aggradational mode; this switch of depositional modes corresponds to an Exxon-model Type 2 sequence boundary. Construction of genetic stratigraphic sequence diagrams and comparison to Exxon's coastal onlap curves across different areas of the Oligocene coast show that the effects of local depocenters (sediment influx) may mask eustatic effects. Only truly regional events, such as the middle Vicksburg transgression and the basal Vicksburg seaward shift in coastal position, correlate across the coastal plain and may result from a eustatic change.