Royalty Lease Evaluation Distillation Data an Underutilised tool to aid Geochemical Analysis, an example from Petrotrin’s Soldado Fields

Authors : 1) Simon Paul ; UTT 2) C. Archie and N. Gallai-Ragobar; Petrotrin

The giant Soldado oil field complex discovered in 1954, has approximately 980 wells and has produced close to one billion barrels of oil to date from seven fields. Formations penetrated in this area range from the Upper Cretaceous Gautier to the Pleistocene Talparo Formation.
Geochemical research by Kuarsingh (1986), Rodrigues (1987, 1988, 1993), Talukdar et al (1990), Requejo (1991), Persad et al (1993), Requejo et al (1994), Baseline/DGSI (2004) indicated that most of the hydrocarbons in Trinidad have been sourced from Type II kerogens contained in Cenomanian- Santonian and Aptian aged marine shales of the Naparima Hill and Gautier Formations respectively, Maastrichtian and Tertiary sediments are dominated by mainly woody and coaly kerogens. These studies have shown there is biodegradation of oils, re-migrated non associated light hydrocarbons, unaltered oils and light oil recharge.
Royalty Lease Evaluation (RLE) analysis of crude oils consists of Specific gravity / API, viscosity, % Sulphur,% of products by distillation. Analysis of the products was performed on at least 600 wells. Conclusions similar to that derived by conventional geochemical methods can be arrived at from this legacy data
A later charge of light hydrocarbons mixing with an emplaced biodegraded oil is evidenced by a phenomenon called the Gas Oil Anomaly. Essentially this is the absence of any gas oil fraction combined with the presence of light hydrocarbons in the distillation data. Comparison to the gas chromatographs will be used to verify this observation. It will also be suggested that presence of this late charge of light hydrocarbons has been the key factor in the prolific production from the Soldado reservoirs.
There are also non associated light hydrocarbons where some of these oils exhibit unusual compositions. There are mixtures of light oil or condensate and early emplaced biodegraded oil. These have resulted from a process termed phase separation or evaporative fractionation. A late charge for the light hydrocarbons is suggested as they appear relatively unaffected by biodegration. Post-emplacement biodegradation is observed and documented, this has happened by either the action of aerobic or anerobic bacteria in reservoir waters.
The number of unaltered oils is small in these field.
Additional analysis of the light oil and gas oil fractions of a crude oil will reveal properties and characteristics suggesting there were different sources for both the originally emplaced oils and the later charge of light hydrocarbons. The data also shows there is no correlation with API Gravity, oil viscosity, sulphur content and depth of the reservoirs in Soldado Field.