Variability of Sandstone Architecture and Bypass Systems of the Miocene Oakville and Lower Lagarto Formations in the Carancahua Bay Area, Texas Gulf Coast

Rattanaporn Fongngern¹ and William A. Ambrose²
¹Department of Geological Sciences, University of Texas at Austin, University Station C9000, Austin, Texas 78712–0254
²Bureau of Economic Geology, University of Texas at Austin, University Station, Box X, Austin, Texas 78713–8924

The 3400-ft lower Miocene Oakville and the basal middle Miocene Lagarto Formations contain five type-1 depositional sequences of duration 0.6–2.5 m.y. Four regressive units of the Transgressive-Regressive sequences were used to produce gross sandstone maps. Integration of well log patterns, sandstone dispersal trends and seismic stratal slices was used to interpret depositional environments. Lowstand systems tract (LST) deposits are mainly incised-valley fill (IVF) facies. Transgressive systems tracts (TSTs) are composed of retrogradational barrier/tidal-inlet, lagoonal, and reworked deltaic facies. The lowermost highstand systems tract (HST) deposits are predominantly fluvial-dominated deltaic facies, whereas other HSTs are represented by waveinfluenced deltaic and strandplain systems. Gross sandstone maps of the selected systems tracts reveal variable coastal depositional systems, in contrast to previous work interpreting these deposits as solely aggradational shorezone environments. Additionally, regressive-unit gross sandstone maps illustrate progradational shelf systems and northeastward shifting of lower Miocene depocenters. During LSTs, sediments bypassed the shelf through incised valley systems and possibly formed submarine fans that would commonly become good reservoirs in downdip depocenters. Hence, the presence of incised valley facies in the study area is inferred to be predictively linked to downdip slope and basin-floor exploration targets.

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012