A regional chemostratigraphically-defined correlation framework for the late Triassic TAG-I Formation in Blocks 402 and 405a,Algeria

摘要

The Triassic Argilo-Greseux Inferieur Formation(TAG-I)is one of the principal hydrocarbon reservoirs in the Berkine Basin of Algeria.Sedimentological studies have shown that it exhibits marked spatial and temporal facies variations on both a local field scale and a regional basinal scale.This variability,combined with a lack of diagnostic flora and fauna,makes regional correlation within the unit difficult.In turn,the lack of a consistent regional stratigraphic framework hampers the comparison of the various correlation schemes devised by operators in the basin.Contrasting the TAG-I in Blocks 402 and 405a exemplifies the problems encountered when attempting regionally to define a correlation framework for the interval.Between these two blocks,a distance of approximately 200 km,there are marked changes in the style of deposition from sand-dominated,proximal fluvial systems in the SW(Block 405a,MLN,MLC,KMD and MLNW fields)to a more distal,more clay-prone system in the NE(Block 402,ROD/BRSE/BSFN,SFNE and BSF fields).A chemostratigraphic study of the TAG-I in these two blocks has allowed a four-fold correlation framework to be defined,where each chemostratigraphic package has distinctive geochemical features.Chemostratigraphic Package 10,the oldest unit,lies above the Hercynian Unconformity,but beneath a geochemically identifiable hiatal surface.Chemostratigraphic Package 20 lies above the hiatal surface but is separated from the overlying packages by a mineralogical change identifiable in both claystone and sandstone geochemistry.Chemostratigraphic Packages 30 and 40 are chemically somewhat similar,but are separated by a regional event interpreted as a period of dolocrete and lacustrine development.By combining the geochemical differentiation of the units and recognition of their stratal boundaries,it is possible to define a correlation for the TAG-I between Blocks 402 and 405a.The proposed correlation between the two blocks suggests that the northern parts of Block 405a may have been occupied by a spur or subsidiary channel from the main SW-NE-trending fluvial system,resulting in one of the chemically defined packages being demonstrably absent in the MLNW,MLN,KMD and MLC fields when compared with the other areas of the study.