Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Geological and Geophysical Characteristics of the Ultra-Deep High-Porosity Carbonate Gas Reservoirs, Puguang Gas Field, Sichuan Basin, China
(1) Department of Geology & Geophysics, Texas A&M University, College Station, TX.
(2) Sinopec Southern Exploration Company, Chengdu, China.
(3) Sinopec, Beijing, China.
The Puguang Gas Field contains one of the deepest carbonate gas reservoirs discovered in the recent history of hydrocarbon exploration in China, which is situated at a depth of >5 km in the mature Sichuan basin. One of the important features of this deep field is that there exist thick and high-porosity (10-28%) reservoir zones but they are highly heterogeneous with permeability differing by 4 orders of magnitude. The existence of such high-porosity reservoir rocks at so great a depth presents profound challenges to the conventional concepts and methods of geology and geophysics alike. Moreover, a better understanding of the geological and geophysical signatures of these high-porosity carbonate rocks at great depths could shed light for on-going and future deep exploration.
Through detailed analysis of hundreds of core photographs and thin sections together with a complete suite of well logs, we find that the main pore types in the high-porosity rocks in the Puguang gas field are oolitic dolomites with moldic porosity and sucrosic dolomites with intercrystalline porosity. Previous work shows that oolitic dolomites were formed by supergene and selective dissolutions whereas the sucrosic dolomites were formed through recrystallization after burial dissolution and thermochemical sulfate reduction. In addition, we observe that both dolomites may have the same porosity but they differ considerably in both sonic velocity and permeability. For a given porosity of ~20%, oolitic dolomites with moldic porosity may have a sonic velocity of >5.5 km/s and a permeability of less than 1 mD, whereas the sucrosic dolomites may have a sonic velocity as low as 4.3 km/s and extremely high matrix permeability of over 3300 mD. In the studied reservoirs, rocks with porosity of >20% are usually dominated by sucrosic dolomites.
Based on a rock physics model, a pore type quantifier referred to as the frame flexibility factor is introduced to distinguish the sucrosic dolomites from the oolitic dolomites with molds. A frame flexibility factor of >3 indicates mostly oolitic molds with porosity ranging 3% to 20% and permeability less than 1 mD. A frame flexibility factor of <3 indicates mostly sucrosic dolomites with porosity from 4% to 28% and permeability up to 3355 mD. The frame flexibility factor can be further estimated from sonic logs and pre-stack seismic data for high-quality reservoir prediction in the studied and other similar deep basins.