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Sequence Stratigraphy, Sandstone Architecture, and Depositional Systems of the Lower Miocene Succession in the Carancahua Bay Area, Texas Gulf Coast

Fongngern, Rattanaporn *1; Ambrose, William A.2
(1) Geological Sciences, University of Texas at Austin, Austin, TX.
(2) Bureau of Economic Geology, University of Texas at Austin, Austin, TX.

This study defines depositional environments and constructs the sequence stratigraphic framework of the Lower Miocene Oakville Formation and the basal part of the Middle Miocene Lagarto Formation in the Carancahua Bay area. Our data set includes a 3-D seismic volume covering 200 sq. miles and 45 well logs. The 1600 ft thick study interval was divided into five type-1 depositional sequences encompassing 0.6-2.5 My. Systems tracts were identified by stacking patterns and bounding surfaces. Sequence thickness increases from sequence 1 to 3 and decreases from sequence 3 to 5. This implies an initial increase in the rate of accommodation generation relative to sediment supply followed by reduction in accommodation space relative to sediment supply. Four regressive units (R-Unit) of the T-R sequence model were also defined. Net sandstone maps for the R-Units all exhibit dip-aligned patterns, representing delta-plain channels and delta-front depocenters.

Integration of well log patterns, sandstone dispersal trends from net sandstone maps and seismic stratal slices were used to interpret depositional environments in each sequence. Lowstand Systems Tracts (LST) deposits are represented mainly by incised-valley fill facies. Transgressive Systems Tract 1 (TST1) is composed predominantly of retrogradational barrier/tidal-inlet facies, whereas other TSTs contain lagoonal and reworked deltaic systems. Highstand Systems Tract 1 (HST1) is composed of fluvial-dominated deltaic systems, whereas other HSTs represent wave-influenced deltaic and strandplain depositional systems. Net sandstone maps in these HSTs reveal dynamic variations in coastal depositional systems, in contrast to previous work that interpreted these deposits as simple, aggradational shorezone systems. Additionally, the R-Unit net sandstone maps illustrate progradational-shelf systems and display northeastward shifting of lower Miocene depocenters.

During LSTs coarse-grained sediments bypassed the shelf through incised valley systems to a downdip depocenter. Destructive processes associated with TSTs reworked sandy sediments. More sandy sediments were stored on shelf as deltaic and strandplain deposits during HSTs. Submarine fans formed by sediments transported through incised-valley systems and delta fronts are commonly good reservoirs. Hence, the presence of these depositional facies in the study area is inferred to be genetically linked to downdip exploration targets.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California