Geochemical Characteristics of Bitumens and Seeps from Tanzania

MPANJU, FLORA, Tanzanian Petroleum Development Corp., Dar Es Sallam, Tanzania, and PAUL PHILP, University of Oklahoma, Norman, OK

A number of bitumen extracts from prospective source rocks and oil seeps of potential oil-producing areas in Tanzania have been characterized by a variety of geochemical techniques. The data obtained from this study have provided additional insight into the source rock potential of these areas. However, in this paper it is proposed to discuss in detail the results from two of the more unusual samples in this region, namely Wingayongo and Pemba. The Wingayongo bitumens isolated from an exposed Neocomian-aged sandstone, possibly a paleoreservoir, are almost totally devoid of n-alkanes and steranes and dominated by hopane-type biomarkers with the so-called immature bb-stereochemistry at the C(17) and C(21) positions. There is no typical evidence of biodegradation having occurred leading o the proposal of an unusual source material or maturity history for this sample. The Pemba seep samples were also characterized by relatively high concentrations of hopanes with the immature stereochemistry at the C(17) and C(21) positions and a virtual absence of n-alkanes and steranes. Furthermore, the isotopic compositions of the saturate and aromatic fractions were exceedingly light with values of -47 and -59%, respectively. The aromatic fractions contained relatively high concentrations of hopanic acids, with the immature stereochemistry at C(17) and C(21). On the basis of these data, it is proposed that the seeps in the Pemba region are not true oil seeps. Rather they are formed as a result of extremely high levels of bacterial activity with the bacteria utilizing natural gas in t e region as the substrate. The net result is a material referred to in other areas of the world as paraffin dirt whose occurrence results from extensive microbial activity in the region and not directly from seepage of products having a thermal origin.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)