Prediction of Sweet Spots in Complex Paleokarst Reservoirs in Ordovician, Lunnan Area, Tarim Basin, China

Abstract

This study shows how a highly variable hydrocarbon production pattern can be explained and new infield drilling wells can be predicted in a deeply buried, extremely complex paleokarst system on the basis of integrated investigation of outcrop, core, wireline log, and 3D seismic data. A high-quality 3D seismic data set acquired in Lunnan field, Tarim Basin, western China, serves as a key component for this study by providing a rare opportunity to characterize in great detail the 3D geomorphology of a paleokarst drainage system and the related collapsed paleocave system. Interpreted geomorphologic and depositional elements for the drainage system include fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. Interpreted features in the collapsed paleocave system include chaotic reflection patterns related to chaotic breccia, bright spots indicating cave sediments, faults, and sag structures. Similar features have been observed in regional outcrops and conventional core sections. Post-karst faults that transect the karstified carbonate rocks and overlain clastic sequences can also be mapped in detail. By analyzing accumulated productions in more than 50 wells drilled in the 3D survey area, we conclude there are five factors that might have significant controls on reservoir distribution, heterogeneity, quality, and pay potential: (1) distribution of collapsed paleokarst systems is related to direction and distribution of surface drainage channels (valleys); (2) good reservoirs are indicated not only by bright spots, but also by highly distorted reflection patterns; (3) surface fluvial valleys provided poor sealing to hydrocarbon pools in karstified rocks; (4) post-karst faulting may or may not damage hydrocarbon accumulations; and (5) highly-productive wells seem to correlate to the extensional fractures developed in the outer-tension zone in collapsed paleocave systems. These results can be useful in future drilling campaign in the field.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014