Prospect Generation Using Geology, Geophysics, and Geochemistry

In order to find oil and gas the explorationist needs to elucidate the components of a petroleum system consisting of:

source
migration pathway
trap
seal
complementary timing of events

These components are key to assessing whether a container will contain a hydrocarbon charge.

Assessing seismic data for possible traps is a key to delineating prospects for drilling. But which prospect to drill? The largest trap or the largest trap with hydrocarbon charge, i.e., the one with oil or gas?

Seismic data can be utilized with other geological and geochemical data to assess the potential for hydrocarbon charge in a given trap. Using basin modeling and compositional kinetic data allow the prediction of hydrocarbon charge and expected yields. But how is the input data for these models derived?

Figure 1. Seismic line with possible well sites based on trap sites and trap size. For example a stratigraphic test drilled at shot point 225 yielded a DST oil. This well was used to optimize a well model using geological and geochemical parameters to constrain extent of erosional events, heat flow through the sedimentary column, and the timing of hydrocarbon generation as well as the type and distribution of products generated using geochemical inversion.

Principal of Geochemical Inversion:

using available geochemical data to predict otherwise unknown oil and rock characteristics (Bissada, 1991).

An example where the following data are available may be helpful to illustrate the principles and techniques of geochemical inversion:

DST oils were recovered in several nearby wells from different horizons
the basin type is well known is known

How can geochemical inversion be applied to evaluate possible prospects?

Recovered oil samples can be used to make inferences regarding source type, depositional environment, and lithofacies. These data can be used to more accurately infer the timing of hydrocarbon generation and the type and quality of hydrocarbons expected.

For example one DST oil has the whole oil GC and terpane biomarker trace shown in Figures 2a-c.

Figure 2a. Light hydrocarbon fingerprint from whole oil gas chromatographic (GC) analysis showing low cycloalkane content and high aromatic content characteristic of carbonate sourced oils.

Figure 2b. C8+ fingerprint from whole oil gas chromatographic (GC) analysis showing pristane-to-phytane ratio less than 1 and waxy nature characteristic of carbonate sourced oils.

Figure 2c.Terpane biomarker (191 m/z) ion chromatogram showing C35 hopane prominence, high C29 hopane content, relatively low tricyclic terpane content, and C24 tetracylic terpane > C26S tricyclic ratio indicative of carbonate sourced oil.

Figure 3.Source lithofacies inference from oil geochemistry (Hughes et al., 1995). The source of this particular oil is inferred to be marine carbonate.

Knowing that an oil is sourced by a marine carbonate source rock is extremely important in assessment of the timing of hydrocarbon generation. Marine carbonates, which are generally sulfur-rich, will decompose and form oil and gas under lower thermal stress than marine shales, lacustrine, evaporitic, terrestrial and other source rock types

Figure 4. Variability in the rate of organic matter decomposition into oil and gas based on measured bulk kinetic parameters.

By inverting these geochemical data, the characteristics of the source rock can be inferred, and reasonable models of hydrocarbon generation can be utilized to determine the likelihood of hydrocarbon charge in a given prospect.

The timing of hydrocarbon generation may be determined more directly by determining kinetic parameters on oil asphaltenes (di Primio et al., 1999). These measurements on oil or seep asphaltenes preserve the ability to model the earliest generated products so the full content of the oil generation window can be accurately assessed.

Understanding depositional environments, basin types, and application of geochemical inversion can be used to high-grade prospects in a variety of ways. Rifting systems are generally conducive to the formation of lacustrine source rocks, which have high potential for oil. However, these sources usually have a homogenous organic matter composition and the timing of generation is quite different from other kerogen types (see Figure 4). These types of organic matter form oil over a very narrow temperature range due to their single energy bond strengths.

Generation occurs over a narrow temperature window and, thus, accurate modeling of temperature is crucial in the assessment of hydrocarbon charge. Evaporites with source potential often have similar characteristics.

Other Type II marine shales can be quite variable in their rates of hydrocarbon formation depending on both oxygen and sulfur contents.

Type III organic matter has very broad decomposition profiles and will generate hydrocarbons over a broad temperature window.

Detailed evaluation of petroleum systems can be derived from inversion of available geochemical data on oil samples. These data allow accurate calibration of models, which may then be extrapolated to nearby prospects for evaluation of hydrocarbon charge. This assessment includes the timing of trap formation and hydrocarbon generation as if the trap is not formed during generation and expulsion, no commercial hydrocarbons will be recovered.