Shallow Gas Reservoir in a Pleistocene Transgressive Sand Sheet Developed During the Drowning of Retrograde Delta Lobes, Louisiana Continental Shelf
Lillian G. Flakes and Richard H. Fillon
A strongly negative seismic amplitude discovered in our 3-D data set at 450 ms has been tied to the interface marking the top of a thick sand section ca. -1300 ft sub-sea. Sequence stratigraphic and stacking pattern analysis of SP and GR logs point to a Late Pleistocene low-stand delta bar origin for the thick, blocky sands in the lower part of the section. Resistivity data shows the delta bar sands are wet with salt water while an uppermost, thin sand member, capped by shale constituting a notable flooding surface, and potential vertical seal, exhibited a high resistivity signature. With other evidence, this is considered to reflect the presence of free gas in the sand's pore spaces. An amplitude extraction made to-evaluate the reservoir potential of the gas-charged sand member revealed a pattern consistent with three, deltaic lobes aligned along a former drainage axis. The mapped features are considered the result of retrograde delta migration and geomorphic evolution in response to rising sea levels late in the low stand. The upper, gas-charged sand member was interpreted, based on modern analogs, as a transgressive sand sheet containing a combination of facies related to the sub-environments of delta lobe destruction and flooding during rapid marine transgression, e.g.: re-worked barrier island; marine sand shoal; and, inner neritic shelf sands. The Chandeleur Islands and Ship Shoal are modern examples of these features. Because of the relatively thin but widespread character and good sand quality expected for a transgressive sand sheet, this prospect was selected as a low- risk, low-cost candidate for horizontal drilling and completion. The strength of the mapped amplitudes is assumed to vary in response to differences in the below tuning thickness of the sand. Highest amplitudes are predicted where the transgressive sand sheet is thickest and the facies "cleanest" . Modern transgressive deposits suggest a P-90 sand sheet thickness on the order of 20 ft. Our pilot hole, drilled in preparation for the first horizontal well, encountered 18 ft. of sand and 11 ft. of gas on water.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California