What’s new in exploration

 Vol. 233 No. 8

Petroleum geochemistry is taking compositional analysis of petroleum into the exploration realm. Positions are opening up for geochemists and petroleum systems analysts to handle burgeoning work in unconventional plays. Evaluating new plays involves a thorough understanding of source rocks and petroleum generation and maturity, using molecular and isotopic fingerprinting to identify migration paths of different petroleum phases in reservoirs. It requires hands-on work with rock and fluid samples, which is seldom among explorationists’ tasks today.

Traditional geochemistry applications are source rock identification and molecular analysis. Source rock analysis remains the largest single use, providing petroleum composition information used in fluid correlations, as well as maturity, timing of charge, hydrocarbon volume and quality.

Techniques include whole-oil molecular analysis and compound-specific isotope analysis of carbon (CSIA-C) and hydrogen (CSIA-H) in natural gas and oil. Isotopic signatures can validate multiple source rocks or multiple organic facies with a single source. We can even track oil to its biological precursors.

Expertise. Joe Curiale, a Chevron petroleum geochemist, recently presented results of a survey of nine majors and independents, and two service companies in Houston. At the end of 2009, these companies employed 45 full-time petroleum geochemists. In two-and-a-half years, they have added 15, a 30% increase. This does not include what may be an even-larger group of petroleum geochemists working as consultants. Curiale attributes the growth to industry expansion and more unconventional exploration targets.

Petroleum biomarkers. Petroleum biomarkers are complex, carbon-based molecules derived from formerly living organisms—“molecular fossils.” They are diagenetic alteration products of compounds, retaining all or most of the original carbon skeleton of the natural product. These organic compounds are found in crude oils, bitumens and petroleum source rocks. They are typically analyzed using gas chromatography and mass spectrometry.

Biomarkers are used to unravel the stratigraphic origin and migration pathways of petroleum. They are used to determine the relative amount of oil-prone vs. gas-prone organic matter, age of source rock, environment of deposition, lithology, and thermal maturity of source rock, all key to effective basin modeling.

Chromatography. Traditional bio-marker analysis has taken a leap forward with the introduction of multi-dimensional gas chromatography, a powerful technique that can characterize complex samples.

Two-dimensional gas chromatography, GCxGC, is a useful improvement that separates minor components hidden in the background matrix, allowing more accurate identification. All effluent from the primary column passes through the secondary column, and chromatographic resolution is improved, using two orthogonal separation phases.

The GCxGC modulator has recently been coupled with time-of-flight mass spectrometry (TOFMS), although TOFMS has been available since the late 1950s. TOFMS is a mass spec method that separates ions by their masses, as they travel at different speeds within the instrument. TOFMS instruments are extremely sensitive and provide full spectra in seconds.

Adding full mass range TOFMS to 2D gas chromatography allows spectral fingerprinting of all components, and is known as GCxGC-TOFMS. The first instruments to examine fine-grained source rocks just came out a few years ago. Curiale points out that the technique provides “previously unavailable information about biological input, depositional setting, the effect of non-hydrocarbons on physical properties of petroleum, and staged loss of hydrocarbons during biodegradation.”

CSIA. Compound-specific isotope analysis measures the isotopic composition of individual organic compounds and molecules. CSIA-C involves stable isotopes of carbon, 13C and 12C. CSIA-H involves stable isotopes of hydrogen, 2H (D, deuterium) and 1H (protium). Samples of organic compounds are dissolved and injected into a gas chromatograph coupled to an isotope-ratio mass spectrometer (GC-irms). CSIA-H is becoming a routine analysis tool, and can show mixed sources.

Well-site geochem. Weatherford has developed a new set of field-deployable geochemical tools to provide real-time data. Organic measurements of total hydrocarbons, TOC, and fluid quality are obtained with the advanced gas analyzer and the source rock analyzer (SRA). Advanced gas extraction uses membrane technology to deliver results in 55 sec., and the data correlate with downhole fluid analysis (RFT/DST sampling). A semi-permeable membrane on a probe extracts and analyzes hydrocarbon (C1 – C8, benzene, toluene) and non-hydrocarbon (CO₂, N2) gases dissolved in the drilling fluid. The company’s mobile SRA pyrolysis instrument uses drill cuttings to measure residual oil content in source rock; remaining potential for hydrocarbon generation; and thermal maturity. The tools are housed in a mobile lab.

Developing concepts. Massive amounts of geochemical data exist, much of it public but archived on paper. If these datasets were digitized and made available through a distributed relational database, they could be mined to extract useful patterns, says Curiale. One application is the identification of sweet spots in plays.

Geochem data could also be used to predict source rock deposition, using paleoclimate data and modeling restricted circulation. Petroleum geochemistry advamces help explorationists unlock increasingly challenging prospects. Do we employ enough geochemists?