|
Abstract:
|
The Eagle Ford is a well -known source rock for both sandstone (Woodbine ) and carbonate (Austin and Buda ) hydrocarbon reservoirs in East and South Texas . Recent discoveries have demonstrated that source rocks , such as the Eagle Ford , are capable of producing significant volumes of gas and oil . At the same time , variations in well producibility indicate that these rocks , like conventional reservoirs , display considerable geological heterogeneity . Yet , only limited research has been published on the subsurface stratigraphy and character of Eagle Ford facies . Understanding the types , controls , and distribution of these heterogeneities requires in -depth rock -based studies .
In order to characterize Eagle Ford facies , 27 cores from 13 counties were investigated for rock textures , fabrics , sedimentary structures , and fossil assemblages . These studies were supported by light and electron microscopy as well as analysis of elemental chemistry and mineralogy . Regional subsurface stratigraphic correlations and facies distributions were defined using wireline logs calibrated from core studies .
In South Texas , the Eagle Ford Formation was deposited during a second -order transgressive /regressive cycle on the flooded , oxygen -restricted Comanche Shelf . Nine depositional facies consisting predominately of organic -rich , fine -grained (5 .0 % TOC ) to coarser -grained (3 .05 % TOC ) fabrics were identified . Facies developed in low -energy environments episodically interrupted by higher -energy , event sedimentation (current winnowing , cohesive and non -cohesive density flows , and turbidity flows ) . Locally , these rocks show evidence of early diagenetic recrystallization of calcite .
Concurrent water anoxia and organic matter preservation persisted locally into later Austin deposition , resulting in formation of a three -fold division of the Cenomanian -Coniacian Eagle Ford Formation . Common facies of lower and upper Eagle Ford members include (1 ) unlaminated , fissile , clay - and silica -rich , organic -rich mudrocks , (2 ) laminated , calcareous , organic -rich mudrocks , and (3 ) laminated , foraminifera - and peloid -rich , organic -rich packstones . The transitional Eagle Ford member consists of highly -cyclic (1 ) ripple -laminated , organic -rich wackestone (cycle base ) and (2 ) burrowed , organic -lean lime wackestones (cycle top ) . Transitional Eagle Ford facies developed in oxygen -restricted , basinal depositional environments as distal equivalents to burrowed , foraminiferal lime wackestones of the Austin Formation .
Facies complexities in the Eagle Ford stem from complicated and interrelated processes of sediment production and distribution , diagenesis , and water column chemistry . Integrated core studies shed light on both controls of facies formation and their spatial distribution . These findings provide a framework for upscaling the fine -scale , heterogeneous character of shelfal Eagle Ford mudrocks ; thus allowing development of predictive models into the distribution of key reservoir properties in the subsurface . |