Depositional Environments and Porosity Evolution, El Abra Limestone (Cretaceous), Mexico: ABSTRACT

Abstract

Reservoir quality in the middle Cretaceous Golden Lane oil field, Mexico, was enhanced by freshwater dissolution during subaerial exposure in the Late Cretaceous and early Tertiary. Study of the outcropping equivalent El Abra Limestone in the type area demonstrates the presence of repeated emergence and submergence and the formation of subaerial discontinuity surfaces. Physical correlation has been established between El Abra quarry sections. This dovetails with a corresponding correlation of discontinuity surfaces to provide a series of reliable horizons on which to construct a shelf-edge model. Near back-reef rocks are characterized by thin (3 m) shoaling-up sequences usually capped by thick (to 10 m) supratidal and island sequences. Storm washover deposits with strong calcrete overprint are interbedded with penecontemporaneously dolomitized supratidal rocks. Freshwater dissolution during emergence produced voluminous moldic and vuggy porosity, subsequently reduced by interlayered marine internal sediment and a radiaxial fibrous cement mosaic. Platform-interior rocks are characterized by thick subtidal deposits and 3 to 5 m shoaling-upward sequences attributable to tidal flat progradation in the lee of shelf-edge islands or the migration of tidal channels. Paleosols and karstic surfaces are present at the tops of several cycles. Moldic and vuggy porosity developed during sub erial exposure was reduced by vadose and marine internal sediment. Radiaxial fibrous cement mosaics are not present. Porosity development in the El Abra type area is a result of synsedimentary emergence. Porosity was of local extent and largely occluded by processes active during succeeding sedimentary episodes. Post-El Abra freshwater leaching appears to have been minor. Contrasts in reservoir development between the Golden Lane and the comparatively tight El Abra type area may reflect differences in later exposure. End_of_Article - Last_Page 607------------