Lithofacies types and influence on bitumen saturation in x-horizon of the Nigerian tar sand deposits.

Abstract

Bitumen saturation, particle size distribution and mineralogical analyses were carried out on some tar sand deposits in parts of southwestern Nigeria with a view to determining their sedimentological properties and the degree of bitumen saturation of the respective lithofacies. Fifty one fresh samples collected at 1 m intervals from 13 different locations were prepared for further studies by soaking in toluene, to extract the bitumen from the reservoir sands and subsequently subjected to granulometric analysis. The sands were air-dried and dry-sieved to establish the grain size classes for the different locations. The dry samples were further analyzed petrographically to deduce the textural characteristics and transportation history. Moreover, heavy mineral analysis was carried out to determine the sediments' maturity and their provenance. From the textural study, five different lithofacies were established namely: very fine sand, fine sand, medium sand, coarse sand and very coarse sand. Results showed that the samples from some locations were well sorted, while others ranged from poorly sorted to well sorted with average standard deviation range of 0.32 – 1.28Φ. The average graphic mean of 1.24Φ and average standard deviation value of 0.91Φ revealed that majority of the sands were medium grained. Most of the sediments were leptokurtic while others were mesokurtic to platykurtic. The skewness of all the samples varied from very fine skewed to very coarse skewed. The grain shape ranged from angular to well rounded. The heavy mineral analysis result showed that the sediments were mostly mineralogically immature except for those from Idiobilayo, Loda and Trianga. The average percentage bitumen impregnation for Imeri (coarse sand) was 15.5%; Okerisa (medium sand) was 25%; while that of Tirianga (fine sand) was 30.60%. The results generally showed that fine-grained facies had higher tar saturation than the medium and coarser grained sizes, which suggested that bitumen saturation in the study area was influenced by particle size.

Keywords: Bitumen, Tar Sand, Provenance, Maturity, Sorting, Lithofacies.