Diagenesis and its impact on reservoir quality in a Triassic Sandstone, Berkine basin, North Africa

Abstract

The TAGI reservoir was deposited in a Triassic braided fluvial system; it is widely distributed across the Berkine Basin of northern Africa. Reservoir quality across the basin is generally lower at depth due to compaction. However, production tests in the study area, which is located in the northern part of the Berkine Basin show that the TAGI in a deeper area has somewhat higher production rates in the lower part of the TAGI reservoir where it directly overlies the Hercynian Unconformity. This improvement in reservoir quality is thought to indicate a lack of compaction or some other diagenetic association. The mineral compositions, diagenetic components, intensities and sequences were analyzed using a combination of petrography, XRD, SEM, core porosity-permeability, well testing and conventional well log data. Much of the variation in production rates and reservoir quality is a result of the varying development of authigenic phases in the TAGI. The two most significant factors associated with improved reservoir quality at depth are the presence of chlorite rather than illite and dissolution porosity. That is, proportions of secondary dissolution pores and levels of grain-coating chlorite are more abundant in high flow wells, while illite, mixed-layer illite/smectite, anhydrite and carbonate cements are more abundant in low flow wells. The clay mineral types, chlorite versus illite, can be identified using spectral gamma ray cross plots. Much of the improvement in reservoir quality has occurred in the deeper burial realm where late-stage laterally-flowing highly-saline (dense) acidic basinal waters are being focused into the lower part of the TAGI sand. This is a region of pore-fluid cross-flow directly above the impervious fine-grained sediments that underlie the Hercynian Unconformity.