Mechanically Infiltrated Clays: Recognition and Influence in Fluvial Reservoirs of Reconcavo Basin, Brazil

Marco A. S. Moraes, Luiz F. De Ros

Fluvial sandstones and conglomerates of the Sergi Formation (Jurassic) are the main reservoirs of the Reconcavo basin in northeastern Brazil. These reservoirs contain significant amounts of interstitial detrital clays resulting from early diagenetic mechanical infiltration. The infiltration developed under arid/semiarid conditions, where the lowered water table allowed muddy waters of episodic runoff to infiltrate through the coarse alluvium. The main clay concentrations occurred in the upper phreatic zone and in proximity to sources of influent seepage. It is difficult to identify infiltrated clays in ancient sedimentary rocks due to a lack of well-established petrographic criteria. In this work, a series of petrographic criteria is proposed to recognize these clays in c astic reservoirs. These criteria include the anisophachous coatings of tangentially accreted lamellae and the geometric patterns developed due to shrinkage. The infiltrated clays are among the main controls of reservoir properties, and the horizons of maximum clay concentration are the main internal barriers in most Sergi reservoirs. In general, infiltrated clays damage reservoir quality by creating macroheterogeneities and microheterogeneities, by decreasing recovery efficiency and permeability, and by increasing water saturation. Also, they can cause formation damage either by their swelling properties or through the migration of loose particles left by shrinkage. As demonstrated in the Sergi Formation, infiltrated clays must be adequately identified for the definition of proper procedures for drilling, completion, reservoir development, and EOR programs.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.