Palynology, Palynofacies And Organic Geochemical Analyses Of Cretaceous And Early Paleogene Sediments, Offshore Tano Basin, Western Ghana

Abstract

Palynological analysis from two exploratory and one appraisal wells (Lynx-1X, Dzata-1 and Dzata-2A) and organic geochemical results from two wells (Dzata-1 and Dzata-2A) samples were obtained from the Middle Cretaceous-Early Tertiary of the Deepwater Cape Three Points offshore Tano Basin, Western Ghana. From the rich and well preserved palynomorphs recovered, the First Appearance Datum (FAD) and Last Appearance Datum (LAD) of stratigraphically significant species were used to propose four palynozones (PZ-I to PZ-IV) for the samples. Lynx-1X is Albian-Eocene, Dzata-1 and Dzata-2A wells are dated Albian- Maastrichtian, based on evidence of stratigraphically significant sporomorphs and dinocysts. The sporomorph associations recovered from the three wells exhibit similarity to Cretaceous Phytogeographic Provinces of African-South America (ASA). Sporomorphs recorded are characteristic of Albian-Cenomanian Elaterate Province for the deeper intervals and of the Senonian Palmae Province for the shallower intervals in all the wells. The Late Cretaceous peridineacean assemblage has a lot of similarity with those of Malloy or Tropical/Subtropical suite of Lentin and Williams (1980). Distribution of palynomorphs enabled the identification of two major sedimentary facies: the nearshore and open marine facies. The nearshore facies, concentrated at deeper intervals, are characterized by abundant sporomorphs and peridinoid dinocysts while the open marine facies are dominated by gonyaulacoid dinocysts and are restricted to the shallower intervals. This occurred as a result of marine transgression which flooded the area causing marine sedimentation. Palynofacies analysis carried out under transmitted microscopy defined seven palynofacies associations (PF-1 to PF-7) for Lynx-1X, five palynofacies associations (PT-1 to PT-5) for Dzata-1 and six palynofacies associations (PT-A to PT-F) for Dzata-2A wells. The first palynofacies assemblage in Lynx-1X well, PF-1, reflects deposition in a fluviodeltaic/ nearshore environment under a marginal dysoxic-anoxic basin condition with sediments typical of kerogen type III-IV (gas prone). PF-2 is deposited under a proximal suboxic-anoxic shelf conditions in a marginal marine/nearshore environment and sediments classified as kerogen type II/III (gas prone). PF-3 suggests deposition in a marginal marine to shallow marine environment under distal suboxic-anoxic conditions suggesting kerogen type II>I (highly oil prone). PF-4 is inferred to be deposited under a distal suboxic-anoxic basin condition in a shallow marine environment and sediments characterized by kerogen type II>I (highly oil prone). PF-5 indicates a deposition under a distal dysoxic-oxic shelf conditions in middle-outer neritic environment depicting kerogen type II>I (oil prone). PF-6 suggests a deposition in a shelf to basin transition condition in the inner-middle neritic environments indicating kerogen type III and II (oil prone). PF-7 reflects an outer neritic environment under distal mud-dominated oxic shelf conditions and characterized by kerogen type II/III (gas prone). In Dzata-1 well, PT-1 suggests deposition in a nearshore environment under proximal suboxicanoxic conditions with sediments typical of kerogen type II and III (oil prone). PT-2 indicates deposition in a dysoxic-suboxic conditions in a nearshore environment typifying kerogen type III (gas prone). PT-3 suggests deposition in a marginal dysoxic-anoxic basin condition in a fluvio-deltaic/nearshore environment typical of kerogen type III (gas prone). PF-4 represents inner-middle neritic to outer neritic environment deposited in distal dysoxic-oxic shelf conditions typifying kerogen type II>I (oil prone). PT-5 represents deposition in a nearshore iv environment under marginal dysoxic-anoxic basin conditions which is characterized by kerogen type III (gas prone). PT-A of Dzata-2A well indicates deposition in a nearshore to shallow marine (inner neritic) environment under a proximal suboxic-anoxic shelf condition with typical type II/III kerogen (oil prone). PT-B infers an inner neritic/nearshore depositional environment under dysoxicsuboxic conditions with facies characterized by kerogen type III or II (gas prone). PT-C is deposited under a distal dysoxic-oxic shelf conditions in environments ranging from nearshore/inner neritic to middle-outer neritic characteristic of kerogen type II>I (oil prone). PT-D indicates a nearshore/inner neritic depositional environment under marginal dysoxic anoxic basin conditions and facies constituted by kerogen type III (gas prone). PT-E suggests deposition in inner-outer neritic environment under distal suboxic-anoxic basin condition characteristic of kerogen type II≥I (highly oil prone). PT-F suggests a deposition in a nearshore/shallow marine environment under a distal dysoxic-anoxic shelf environment with facies characterized by kerogen type II/III (oil prone). Rock-Eval pyrolysis and TOC results for Lynx-1X and Dzata-2A wells indicates that most of the analyzed samples are thermally immature to marginally mature and have a good petroleum potential with the ?Turonian-Santonian age samples as a better potential source rocks than the Campanian-Eocene and Albian-Cenomanian source samples. Analyzed samples generally have low kerogen conversion.