Architecture of Collapsed-Paleocave Reservoirs

R. G. Loucks and Paul Mescher

It is important to investigate the architecture of collapsed-paleocave reservoirs at interwell scales in outcrops because reservoir heterogeneities cannot be adequately characterized by cores and log correlation sections. A 3000 foot long quarry wall of Ellenburger strata in central Texas displays the lithologic and pore network heterogeneities at typical well spacings (1300 to 2600 feet).

The quarry wall exposes the transition from stratified host rock into a complex collapsed-paleocave system showing several developmental episodes. This paleocave system has over 2600 feet of laterally continuous chaotic breccias. The dimensions of these breccias are similar as to what is imaged by 3-D seismic over paleocave reservoirs. Collapsed-paleocave reservoirs are not single collapsed passages of tens of feet across, but are homogenized collapsed-cave systems hundreds to several thousand feet across. This concept of scale is very important because collapsed-paleocave systems offer larger exploration targets than individual cave passages.

Collapsed-paleocave systems are complex because they are the homogenization of chaotic breccias and cave-sediment fill from passages, chambers, and shafts and of crackle breccias from roof- and wail-rock and pillars. Pore networks are associated with chaotic breakdown breccias, cave roof- and wall-crackle breccias, and/or clastic sediment fill. Strong heterogeneity within a collapsed paleocave system should be expected. Lateral and vertical changes in collapsed-paleocave-related facies have the strongest effect on reservoir heterogeneity and quality. Within individual facies there can be distinct reservoir quality variation, such as between the cave-sediment fill and associated blocks. Tectonic fractures, however, can interconnect the highly variable pore networks within a collapsed-p leocave reservoir.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California