It is the shape, size, and character of the hydrocarbon trap that occupies the minds of geoscientists, reservoir engineers, and economists alike when evaluating the worth of an undrilled prospect. It is the one technical pursuit that we all have in common.
Acting as a team, the geoscientist is concerned with seismic quality and reservoir and cap rock lithologies, the engineer dwells on fluid properties and recovery factors, and the economist worries about total reserves and production rates. But it is the geoscientist alone who decides whether hydrocarbons have replaced formation water in the reservoir - whether oil or gas will actually be found by the drill.
Geophysicists concentrate their efforts on defining the geometry of traps - nowadays doing this with some success - so that prospects are penetrated at their optimum location. Geologists concentrate on selecting the right basin and the right trap. The responsibility of the geologist is onerous - if there are no hydrocarbons present, then his or her colleagues in the other disciplines have worked for nothing. No amount of technical and economic skills will locate oil that just isn't there.
Almost all basins have a cap rock, most have reservoirs, and many have traps. Although many basins also have source rocks, their mere existence is only part of the story. What type are they? Have they been subjected to the right conditions to have generated hydrocarbons at the right time?
Considering the complexity of source rock evolution, isn't it perhaps surprising that the location of the source rock kitchen, the timing of source rock maturation, the total volume of expelled hydrocarbons, and their migration pathway to the trap do not necessarily form part of the primary data resources available to geologists when evaluating basins and prospects?
It is my belief that the evaluation of source rock evolution should be at the forefront of geological thinking. This evaluation comes from basin modeling - a generic term that refers to analysis of the development of a sedimentary basin through time, and particularly the evolution of basin source rocks. The art of basin modeling, or the creation of a model which simulates the processes operating in a basin, is regarded by many geologists as a specialist activity, when in fact, it is fundamental to exploration.
By way of comparison, reservoir engineers use sophisticated simulation routines to model the movement of oil, gas, and water through a field, and they have powerful computers and software to accomplish this. Clearly, large amounts of data must be manipulated in iterative simulation of the complex physical processes active in a sedimentary basin, and thus geologists also require powerful computer software. But, few geologists have a tool at their fingertips that allows such a simulation to be run effectively. Few have such resources at their disposal.
A criticism often leveled at basin modeling software is that there are so many unknown variables that almost any result can be obtained to suit preconceived ideas. This criticism may have been well-founded, but there are two ways to overcome it:
- Ensure that the software does not allow input of unrealistic physical properties, and that the algorithms that calculate the decompaction/compaction processes are robust.
- Ensure that calibration data are obtained and used whenever possible, so that you are able to compare results with geological reality.
Basin modeling software also needs to simulate the unsteady state processes of deposition, compaction, pressure buildup, and heat flow in different geological terrains. In thrusted regions, for example, where thrust emplacement produces rapid changes in the depth of burial of the source rock, the resultant modifications in thermal properties are extremely complex. For such modeling, very powerful software is required
Just as engineers simulate a field and history match its production history, so should a geologist match porosity and paleotemperature (e.g. from vitrinite or fission track data) in a well with predicted porosity through compaction and predicted paleotemperature through transient heat flow.
Modeling can be 1D or 3D (to better define volumes and migration pathways). Although the 3D approach is a rigorous technique that precisely defines the dynamics of a basin, it requires extensive work to set up a model. With the right software, 1D models for wells are easy to set up and essential for an exploration geologist.
In summary, many geologists are rightly concerned with stratigraphy and sedimentology, but neglect the dynamics of fluid generation and movement. There is no doubt in my mind that understanding the flow of oil and/or gas from the source to a trap - specifically its timing, volume, and direction - is fundamental to the successful search for hydrocarbons. So, why don't we use the best tools available and increase our chances for exploration success?
Dr. Michael R. Smith
Director, TerraMod Limited
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