Abstract: Paleoecology and Porosity Trends of Abnormally High Pressures in Sandstones and Shales
Paleoecology provides a practical basis for predicting the occurrence of abnormally high pressures in the subsurface of South Texas and the Louisiana Gulf Coast. By comparing detailed logs showing the biostratigraphy and paleoecology (including frequency distribution of microfossils) with seismic data and wireline logs, drilling programs can be evaluated and drill-site sample examination can be planned to reduce problems associated with unexpected penetration of abnormally high-pressured sandstones and shale.
In the Gulf Coast region the depositional environment commonly associated with abnormally high formation pressure in wellbores is one of continuous sedimentation of a deltaic sequence; no major erosional unconformities interrupt the depositional sequence. Such deposits, generally encountered downdip, consist of interbedded brackish- and shallow-marine strata indicative of deposition in inner to middle neritic depths of water. In the Rio Grande area, formations of Paleocene and early Eocene age have abnormally high formation pressures also related to sandstones interpreted as offshore bars, as well as deltaic deposits.
Shale diapirs are common in late Eocene and younger formations, but are uncommon or absent in Paleocene and early Eocene beds. These structures, always abnormally high pressured, consist of deeper marine, outer neritic to bathyal shales.
An abrupt regressive sedimentary cycle is common above high-pressure zones, but the paleoecologic evidence for this cycle may be so subtle as to be difficult to detect. The sandstones and shales of this study have no apparent surface communication, and the high-pressured facies seemingly preserved the porosity of the enclosed strata. Porosity-trend plots from acoustical and core data indicate an "arrested" compaction of sands and shales. The general compaction trend of reduced porosity with depth of burial seems to be reversed in abnormally high-pressured facies.
Increase in the contacts between quartz grains and pressure solution along the contacts were observed in the high-pressure zones. The increased grain contacts are cyclic in occurrence and seem to be related to alternating paleoecologic cycles.
Paleoecology and abnormally high pressures may play a more important role in the understanding of sandstone diagenesis than previously has been recognized.
AAPG Search and Discovery Article #90965©1978 GCAGS and GC Section SEPM, New Orleans, Louisiana