Relationship between Paleocave Facies and Associated Pore Networks in Coalesced, Collapsed-Paleocave Systems
R. G. Loucks1 and P. Mescher2
1Bureau of Economic Geology, UT at Austin, Austin, TX
2Geological Resources Company, Plano, TX
Coalesced, collapsed-paleocave systems are characterized by strong spatial complexity that affects the distribution of paleocave facies and associated pore networks. Recognizing how cave systems form and evolve with burial into the deeper subsurface is essential in understanding the distribution of paleocave facies and pore networks.
Six basic cave facies are in a paleocave system: (1) undisturbedhost rock, (2) disturbed-host rock, (3) collapsed roof and wall rock, (4) fine-chaotic breccia cavern fill (transported material), (5) coarsechaotic breccia cavern fill (collapsed material), and (6) cavesediment cavern fill. Pore networks associated with paleocave reservoirs can consist of cavernous pores, interclast pores, crackle breccia fractures, and less commonly matrix pores. Each paleocave facies has a different combination of pore types.
Undisturbed-host rock generally contains minor fracture porosity associated with cavern collapse and minor matrix porosity. Disturbed-host rock commonly contains fractures associated with crackle and mosaic breccias. The megabreccias of the collapsed roof and wall rock can have large interclast pores with the clasts having an overprint of crackle fracture porosity. Chaotic breccias of the cavern fill have interclast pores but these can be filled with cave sediment or cement. Most cave-sediment fills are tight because they have an abundance of terrigenous mud. Reservoirs buried less than 2,000 m commonly have some cavernous pores still open. As the cave system is buried, the pore network evolves from one having mainly cavernous and large interclast pores to one having small interclast pores and crackle and mosaic fractures.
AAPG Search and Discovery Article #90905©2001 AAPG Southwest Section Meeting, Dallas, Texas