Development and Applications of Light-Hydrocarbon-Based Star Diagrams

Abstract

Two star diagrams (multivariate plots in polar coordinates) have been developed to assess subtle chemical differences among oils. The diagrams are based on analysis of C7 hydrocarbons of crude oils separated by gas chromatography. The first of the C7 diagrams is used for distinguishing variations in oils caused by transformation, e.g., water washing, biodegradation, and evaporation. Eight ratios are plotted on this diagram in order of decreasing sensitivity to biodegradation. The ratios are designated Tr1 through Tr8, and the diagram is referred to as the C7 oil transformation star diagram (C7OTSD). A second C7-based diagram consists of five ratios composed of compounds that are very resistant to the effects of transformation, and thus are useful for correlation. These ratios, designated C1 through C5, are plotted on the C7 oil correlation star diagram (C7OCSD). This diagram can be used to correlate both transformed and primary oils based on source-related chemical differences, a task not easily accomplished through most conventional geochemical techniques. For example, condensates often contain no biomarkers due to their advanced maturity and therefore cannot be correlated with lower maturity oils. However, both condensates and oils usually contain light ends allowing correlation by C7-based star diagrams. The division of star diagrams into two types allows one to assess the relative degrees of transformation among a set of oils that have been found to correlate based on the C7OCSD. These microscale correlations detect small chemical differences between oils; and, therefore, allow even slight variations among oils within the same reservoir to be recognized. The technique has many applications that concern both exploration and production. Production-related uses include reservoir compartmentalization studies, investigating reservoir extent and connectivity, determining the source of casing leakage, and determining the extent of commingling of oils from different reservoirs in multicompletion wells (or leaky single-completion wells). Exploration problems that can be addressed are mechanisms and models of field-fill history and evaluation of oil quality in untested pay zones. Most of these problems can be addressed through simple, cost-effective analyses on produced or tested crude oils. For example, a large advantage in production applications is the ability to assess reservoir connectivity without halting production from any well. An additional use of the technique is for investigating contaminants or incompletely refined crudes in refinery streams. In this paper, I present several case studies that have already been done at Saudi Aramco. These studies include three examples of behind-casing leakage, two examples of oil differentiation, and a single example each of correlation of condensates and refinery contaminant tracing.