Eagleford Shale Exploration Models: Depositional Controls from Reservoir Properties
Exploration in the Eagleford Shale follows on recognition that a large percentage of the hydrocarbons generated never migrated. The Eagleford has long been seen as a major petroleum source, principally for the Austin Chalk, but also for oil in the underlying Buda, and in shallower Cretaceous and Tertiary horizons. Previous work has focused primarily on source rock character and documenting source potential. Important now is understanding the Eagleford in a reservoir context and documenting depositional, diagenetic, and structural controls on hydrocarbon saturation, permeability, lateral and vertical variability to optimize leasing, horizontal bore location and completion techniques.
Lower Cretaceous basin geometry exerted a primary control on Eagleford shale deposition, creating local depocenters where primary organic content is higher, controlling downslope current transport thereby producing areas of ponded denity current deposits, bypassed margins, and focusing the location of more distal turbidite fans. In addition, Eagleford depocenter geometry exerted fundamental control on later migration through fault reactivation, which established pathways and barriers to up-dip migration.
This study focuses on an area extending from LaSalle county in the southwest through Atascosa, Wilson, and Gonzales to Fayette county on the northeast and down dip through Webb, McMullen, Live Oak, Karnes, and Dewitt counties. Two fundamental plays are found in this region, separated by the Stuart City Edwards reef trend. Up-dip of the reef margin the Eagleford is normally pressured, primarily an oil reservoir, characterized by significant lateral variability in organic rich shale abundance and reservoir quality related to reef margin controlled depocenters. Down-dip of the reef margin the Eagleford is modestly to strongly over-pressured, primarily a gas reservoir, and characterized by lateral reservoir variability and primary permeability controlled by the location of distal turbidite deposition. Reactivation of faults underlying the Stuart city trend produced barriers to up-dip migration and areas of focused gas accumulation. Corridors between reactivated faults focused oil migration and produced significant variability in oil saturation within the up-dip play. Mapping basic rock properties on well logs has allowed high-grading of Eagleford reservoir properties and suggests the potential to define seismically optimal well locations.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009