Basin Analysis and Petroleum Analysis

Sedimentary basin analysis is a geologic method that examines sediment fill and subsidence to identify the formation and evolution history of a sedimentary basin.

As we explore for hydrocarbons we must ask ourselves many questions:

• Are hydrocarbon source rocks present?
• When were hydrocarbons generated?
• Can we expect gas, oil or both? Along what paths did the hydrocarbons migrate?
• Do traps exist and when did these traps form? And what about the reservoir rocks? Do they have porosity? Permeability?

To answer these questions we use basis analysis. Of the 600 to 700 sedimentary basins around the world only 400 are reasonably well explored. Of these 400 basins only about 150 produce hydrocarbons.

The remaining basins are relatively unexplored for a variety of reasons. Some are victims of politics. Some are inaccessible due to harsh climate or rugged terrain while others are simply under or incorrectly evaluated. We use basin analysis to evaluate the hydrocarbon potential of a basin by assessing certain geological conditions and the timing of geological events. Let’s summarize the basic requirements.

First, we need a diverse assemblage of sedimentary strata so that rocks with reservoir, source and seal properties are in physical and hydrodynamic communication. Next, we need a mature hydrocarbon source to produce petroleum liquids and gases. We need porous and permeable rocks so that hydrocarbons expelled from the source rocks can migrate and be concentrated in reservoir rocks.

Structural or stratigraphic anomalies must be in place to restrict and trap the basin wide flow of petroleum. And we need permeable reservoir rocks overlain by a seal. All of these geological events must occur with the correct timing and in the proper sequence. For example traps must be developed prior to or during the maturation and migration of petroleum. We also need to consider the geologic history of the petroleum after it’s trapped. For example, if the reservoir strata extend to the Earth’s surface, groundwater may mix with and degrade the oil. We must also include non-geological factors on our list of requirements for a successful petroleum venture. We can group these factors into two general categories. Economics and political risk. The economic recovery of petroleum depends on the quantity and quality of the petroleum recovered versus the costs required to acquire, produce, transport and sell it. Of course logistics play an important role and will greatly affect the economics of exploding for, producing and transporting petroleum.

As for political risk governments and laws are certain to change over the length of a production contract. Before committing to an exploration program we must consider and anticipate any potential changes.

Although petroleum and its precursors are dispersed throughout most sedimentary rocks, commercial concentrations of petroleum are found in only a few reservoirs in only some sedimentary basins. Of the approximately 600 to 700 sedimentary basins in the world, 400 are reasonably well-explored, and of these approximately 150 are (or have been) commercially productive. The rarity of commercial quantities of petroleum reflects the low probability that the many geological factors necessary for petroleum accumulation occurred in the proper sequence and under suitable geological conditions. To assess these geologic factors, we must answer a series of questions:

• What types of petroleum were generated, and when?
• Where did petroleum collect after generation, and what migration pathways channeled it there?
• How was the petroleum trapped, and have these traps been breached by later deformation?
• Are economic concentrations of petroleum likely, and can we predict their locations?

We can find the answers to these and similar questions by applying the diverse geoscience and engineering tools and approaches that constitute sedimentary basin analysis.

Figure 1 summarizes the geological conditions and events necessary for an economic accumulation of petroleum.

First, we need a sequence of rocks with reservoir, source and seal properties, and these rocks must be in physical and hydrodynamic communication. There must be a sufficient volume of organic matter to provide a hydrocarbon source. The source rocks must be buried and heated over a sufficient period of time to generate hydrocarbons. We need porous and permeable rocks so that hydrocarbon liquids can migrate from the source rocks and be concentrated in reservoir rocks. Stratigraphic and/or structural anomalies must be in place to create hydrocarbon traps.

These geological events must occur in an appropriate sequence for example, traps must develop prior to or during petroleum maturation and migration. Once trapped, petroleum must not be lost or destroyed through later deformation of the trap or of the petroleum itself. For example, if traps are tilted or broken by faulting, the accumulated petroleum will leak out; and, if maturation continues, oil will be converted to methane gas.

Finally, the economic recovery of oil and natural gas is determined by the quantity and quality of the petroleum relative to the costs required to discover, produce, distribute and sell it. To evaluate the economics of a particular petroleum venture, we must assess its associated geological risks.

Two philosophies have guided the historical development of basin analysis. The first philosophy simply states that being smarter about the geology of an area will decrease our risks and increase our chances of successfully discovering and recovering petroleum. The second, more complex philosophy states that geological processes and events function in predictable ways that lead to predictable results. This philosophy is based on the argument that geodynamic, stratigraphic, hydrodynamic and other process/response systems are interrelated. Consequently, we can develop a reasonable understanding of the geological history of an area even from limited data, and we can predict rock types and fluid contents at locations beyond our control points.

In this second philosophy, we assume that deterministic relationships exist among many disparate and diverse geological elements. Some of these relationships are reasonably well-understood. For example, we recognize the complex interplay between tectonic setting, subsidence history, heat flow and maturation. Other relationships are obscure. Examples include paleoclimate and paleolatitudinal controls on source rock type and quality, and suggestions that accommodation space controls a spectrum of attributes from the stratigraphic architecture to the petrophysics of sedimentary rocks. Still other process/response relationships are unknown but should be expected, because tectonic and geological processes operate within a buffered system containing many complex feedback loops.

In general, we can best understand these observed relationships through an analysis of process/response systems. Rather than presenting a catalog of basin types and their attributes, we summarize the major geodynamic, tectonic, and surficial earth processes along with their most directly linked responses. Where possible, we discuss some of the responses that are less directly linked to individual processes, or that are apparently linked to multiple processes. Using this approach, we develop a foundation that enables us to incorporate new empirical data, new process/response relationships and new classifications of sedimentary basins.

The geosciences are a fragmented group of disciplines with fuzzy boundaries between geology, geophysics, geochemistry and geoengineering. These disciplines are, in turn, divided into specialties, reflecting the tools we use, the aspects of strata we study, or the analytical approaches we use to solve problems. Sedimentary basin analysis integrates these various geoscience specialties. All aspects of basin analysis focus on the strata of the basin, and it is stratigraphy that provides the philosophical foundation for sedimentary basin analysis.

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