Palynology, Palynofacies and Hydrocarbon Potential of Cretaceous Sediments in the Offshore Tano Basin

Abstract

Data from HD-1 well, offshore Tano Basin was subjected to optical and organic geochemical analysis to reveal the palynostratigraphy, palaeoenvironments and hydrocarbon potential of the said well. Organic geochemical studies of Total Organic Carbon and Rock-Eval pyrolysis analysis as well as the microscopic investigation of the 44 slides of ditch cuttings from a depth of 3370ft – 7290ft shows four (4) types of Palynofacies associations in the HD-1 well based on the sedimentary organic matter associations; i. Palynofacies I – Amorphous organic matter dominant ii Palynofacies II – Amorphous organic matter dominant with Palynomorphs iii. Palynofacies III – Amorphous organic matter dominant with opaques iv. Palynofacies IV – Amorphous organic matter dominant with opaques and phytoclast Palynofacies types I, III and IV reflects a Distal suboxic-anoxic deep basin environment for hydrocarbon accumulation and production. (Tyson 1995). Palynofacies type II reflects a Distal dysoxic-oxic shelf environment. Stratigraphically important species of miospores and dinocysts were used to demonstrate palynostratigraphic interpretations which assigned the following ages to the HD-1 well interval; i. Campanian-Maastrichtian to interval 3370ft – 5340ft ii. Turonian–Santonian to interval 5440ft - 7290ft Geochemical analysis and interpretation deductions show that, the HD-I well samples of the Tano basin has a Good to Very Good petroleum potential with a TOC average value of 2.4 wt% which suggests favourable conditions for organic matter production and preservation in the basin (Atta Peters et al., 2016). Modified Van-Krevelen diagram shows that the organic matter of the HD-1 well samples have kerogen types II, II/III and III with few type IV. Kerogen types II and II/III suggest the possibility of the generation of oil and oil/gas respectively. Kerogen type III suggest the ability to produce gas whereas the few type IV Kerogen have no potential of producing any hydrocarbon. In terms of thermal maturity, the samples are represented by Tmax values between 418 – 445oC with an average of 427oC and this shows that majority of the samples are immature to early mature with few samples showing maturity. The source rocks have a good to very good organic richness. The production index (PI) falls between 0.36 and 0.85 with an average of 0.52. This indicates a good part of the samples to be indigenous and the majority being contaminated and nonindigenous.