Source Rock Evaluation

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The evaluation of a rocks past, present, and future potential to generate hydrocarbons by quantifying the elements and processes that control the evolution of an effective source rock in the context of petroleum exploration and exploitation.


Overview

Methods of source rock evaluation have become increasingly valuable as the oil and gas industry continually raises standards on efficiency and effectiveness. By analyzing properties of potential source rocks, E&P companies are able to estimate the total volume and quality of exploitable hydrocarbons in a sedimentary basin, which are determining factors in risk analysis for both conventional and unconventional plays. A general risk analysis:


                        {[ Discovery Probability ] * [( Total Expected Reserves * Wellhead Price ) - ( Operating Cost )]} - {[ Dry Hole Probability ] [ Dry Hole Cost ]}


Thermal maturation, Bissada 1976.


The broadest element in source rock evaluation is the total organic carbon, or TOC, of a rock. TOC is the weigh percent of organic carbon present in a rock which is measured by the amount of CO2 released through combustion. To be considered a potential source rock (a rock capable of generating petroleum) it must contain a TOC of at least 1%, although there have been exceptions.


A potential source rock is considered an effective source rock if it has generated hydrocarbons as a result of thermal maturation, or the effect of thermal stress undergone with increasing burial depth. Once a rock is determined to have adequate TOC, its thermal maturity determines if the physical conditions required for hydrocarbon generation have been met, and to what extent. The figure demonstrates progression of thermal maturity with respect to temperature and time. The timing of a source rocks progression through zones III and IV is critical with regards to exploration and is referred to as the peak oil window. This stage represents the maximum generation of liquid hydrocarbons and the most critical timing for pathways of migration and entrapment to be in place.


Geochemical Evaluation

Sapropel Index, Bissada 1975.

Rock-Eval Pyrolysis

Petroleum generation is a result of the burial diagenesis of organic rich sediments. Thermal stress without the presence of oxygen is responsible for hydrocarbon generation in a process called pyrolysis, as opposed to combustion. As TOC is measured in the lab by combustion, a rock can undergo laboratory pyrolysis to measure its total generation potential. This type of analysis is conducted after levels of TOC (total organic content) have been deemed adequate (appx. >1%). A rock with insufficient TOC will not generate hydrocarbons.

In a pyrolysis analysis, a rock sample undergoes increasing temperature in an inert atmosphere where three peaks of released hydrocarbons can be measured. The first peak (S1) represents the volatilization of any previously generated hydrocarbons present in the rock, given that it has reached adequate thermal maturity. The second peak (S2) is indicative the thermal degradation of any remaining organic material into hydrocarbons. The final peak (S3) is any organic CO2 present in the rock. The temperature at which the S2 peak occurs is an accurate approximation the rocks thermal maturity at depth in the sedimentary basin. In addition to thermal maturation, pyrolysis peaks S1, S2, and S3 yield information about the type of organic material present in the rock. The type of organic material, or kerogen, determines whether generated hydrocarbons will be generally oil (sapropelic kerogen) or gas (humic kerogen), which has huge economic implications for companies interested in exploration and production. Sapropelic and humic kerogen can be broadly differentiated by the relation of the hydrogen and oxygen atomic ratios with respect to carbon. A higher O/C ratio, or oxygen index, is found in gas-yielding kerogens when compared to oil-yielding kerogens. This is related to the degree of anoxia present at the time of deposition. The ratio of S3/S1 peaks and S2/S1 peaks in the pyrolysis analysis are proxies for the kerogens oxygen index and hydrogen index respectively. The figure shows their relation to gas-prone and oil-prone source rocks, as well as their progression of thermal maturation.


Chromatography Fingerprints

A more in depth analysis can be conducted on the gasses released from pyrolysis through chromatography. Chromatograph “fingerprints” are generated based on the relative abundances of light to heavy carbon chains and their distributions in relation to oil-prone and gas-prone source rocks. Pyrolysis gas chromatography yields very specific geochemical data with far reaching applications. For this reason, the technique is most commonly seen in research settings.


Seismic Evaluation

Using seismic data to determine a source rocks TOC content is possible and could potentially revolutionize how oil and gas companies choose to conduct source rock evaluations. Claystone source rocks have shown a predictable reduction in acoustic impedance with increasing levels of TOC. Increasing TOC also intensifies the vertical anisotropy seen in claystones. In seismic data, this produces a high amplitude negative reflection at the top of a potential source rock and a positive high amplitude reflection at the base, which can indicate levels of total organic carbon if calibrated correctly. Furthermore, a difference in amplitude between the top and base reflectors can indicate whether TOC content is increasing or decreasing across the potential source rocks vertical profile.

With localized methods and calibration, seismic source rock evaluation is viable and could prove to be a next innovation in an industry with such high economic stakes.

Source rock seismic reflections, Løseth 2011.

References

A, Al Selwi. “Source Rock Evaluation using Total Organic Carbon (TOC) and the Loss- On-Ignition (LOI) Techniques.” Oil & Gas Research, vol. 1, no. 1, 2015, doi:10.4172/2472-0518.1000105.


Bissada, K. K., 1982, Geochemical constraints on petroleum generation and migration: Proceedings Association of Southeast Asian Nations Council on Petroleum (ASCOPE) 1981, Manila, the Philippines, p. 69–87.


Dembicki, H., K. K. Bissada, L. W. Elrod, E. L. Colling, and R. N. Pheifer, et.al., 1984, "An inter-laboratory comparison of source rock data", Geochimica et Cosmochimica Acta,, vol. 48, pp. 2641-2650.


Katz, BJ (1984): Source quality and richness of Deep Sea Drilling Project Site 535 sediments, southeastern Gulf of Mexico. In: Buffler, RT; Schlager, W; et al. (eds.), Initial Reports of the Deep Sea Drilling Project, Washington (U.S. Govt. Printing Office), 77, 445-450, https://doi.org/10.2973/dsdp.proc.77.111.1984


Løseth, Helge, et al. “Can hydrocarbon source rocks be identified on seismic data?” Geology, vol. 39, no. 12, 2011, pp. 1167–1170., doi:10.1130/g32328.1.


Sayers, Colin M. “The effect of kerogen on the AVO response of organic-Rich shales.” SEG Technical Program Expanded Abstracts 2013, 2013, doi:10.1190/segam2013-0465.1.


Tissot, B. P., and Dietrich H. Welte. Petroleum formation and occurence. Springer, 1984.