Exploration Significance of Unconformity Structure from Subtle Pools in Juggar Basin
Vertical structure of an unconformity can be divided into three layers: basal conglomerate or transgressive sand, weathered clay layer and leached rock. Because the conglomerate and leached rock have high porosity and permeability, they consist good channels for hydrocarbon migration, and weathered clay layer becomes a good seal. However, not all unconformities have identical vertical structures. When the weathered clay layer has a little thickness or limited distribution, the overlying and underlying strata will contact directly, and the lithology is often different. This lithologic difference causes different fluid transporting capacity, and it also determines whether a trap can be formed or not and what kind of trap can be formed. In addition, whether a stratigraphic overlap pool or a lithologic pinch pool can be formed above the unconformity surface was also controlled by the ancient landform of the unconformity surface before its deposition. If the unconformity surface was gentle, the deposited transgression sand bodies or the basal conglomerates would connect with each other, thus a migration channel had been formed. If the unconformity surface was in a slope break in the early transgression, the deposited sand bodies would be covered by the later mudstones, thus to form a stratigraphic overlap trap or a lithologic pinch trap. The leached rocks developed a large number of fractures and secondary pores, and its depth can be several meters up to over hundred meters. On appropriate conditions, the sequences below the unconformity surface can form buried-hill, fractural, structural pools and reconstructed special kinds of pools. Simulation experiments showed that when oil and gas moved along the unconformity, their movements were restricted by the physical properties of the rocks and the slope. The oil and gas could easily get into other layers through the weakest part of the weathered clay layers and migrate within these layers. The overlap traps above the unconformity surface were more favorable than the stratigraphic traps (buried-hill) below the unconformity surface to entrap oil and gas.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009