Combining biomarker and bulk compositional gradient analysis to assess reservoir connectivity

Abstract

Hydraulic connectivity of petroleum reservoirs represents one of the biggest uncertainties for both oil production and petroleum system studies. Here, a geochemical analysis involving bulk and detailed measures of crude oil composition is shown to constrain connectivity more tightly than possible with conventional methods. Three crude oils from different depths in a single well exhibit large gradients in viscosity, density and asphaltene content. The oils were collected with a wireline sampling tool to provide samples from well-defined locations and relatively free of contamination from drilling fluid; the known provenance of the samples minimizes uncertainties in analysis. The detailed chemical composition of almost the entire crude oil was determined by use of comprehensive two-dimensional gas chromatography (GC×GC) to interrogate the non-polar fraction and negative ion electrospray ionization Fourier transform-ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) to interrogate the polar fraction. The simultaneous presence of 25-norhopanes and mildly altered normal and isoprenoid alkanes was detected, suggesting that the reservoir experienced multiple charges and contains a mixture of oils biodegraded to different extents. The gradient in asphaltene concentration is explained by an equilibrium model considering only gravitational segregation of asphaltene nanoaggregates; this grading can be responsible for the observed variation in viscosity. Combining the analyses affords a consistent picture of a connected reservoir in which the observed viscosity variation originates from gravitational segregation of asphaltene nanoaggregates in a crude oil with high asphaltene concentration resulting from multiple charges, including one charge that suffered severe biodegradation. Observation of these gradients having appropriate magnitudes suggests good reservoir connectivity with greater confidence than possible with traditional techniques alone.