ORIGIN AND OCCURRENCE

The declining reserves of light crude oil have resulted in an increasing need to develop options to upgrade the abundant supply of known heavy oil reserves (IEA, 2005; Meyer and Attanasi, 2003; Meyer et al., 2007). In addition, there is considerable focus and renewed efforts on adapting recovery techniques to the production of heavy oil.

Over the past decade, the demand for crude oil worldwide has substantially increased, straining the supply of conventional oil. This has led to consideration of alternative or insufficiently utilized energy sources, especially heavy crude oil to supplement short- and long-term needs. Heavy oil has been used as refinery feedstock for considerable time, usually blending with more conventional feedstocks, but has commanded lower prices because of its lower quality relative to conventional oil.

Obviously, differences exist between heavy oil and conventional oil, according to the volatilities of the constituents. When the lower-boiling constituents are lost through natural processes after evolution from organic source materials, the oil becomes heavy, with a high proportion of asphaltic molecules and with substitution in the carbon network of heteroatoms such as nitrogen, sulfur, and oxygen. Therefore, heavy oil, regardless of source, always contains the heavy fractions, the asphaltic materials, which consist of resins and asphaltenes (Figure 2-1). Removal or reduction of the asphaltene fraction, through deasphalting or leaving these constituents in the reservoir during recovery, improves the refinability of heavy oil.

The significance of the absence of the asphaltene constituents is reflected in the capital and operating expenses required for the recovery, transportation, refining, and environmental mitigation.

Detection of hydrocarbons in the subsurface during exploration takes a number of forms: direct identification of hydrocarbons at the surface, direct identification of hydrocarbon indicators (DHI) in the subsurface, and indirect identification of indicators both at the surface and in the subsurface. Traditionally, oil exploration was primarily conducted by recognizing seeps of hydrocarbons at the surface. The Chinese, for example, used oil (mostly bitumen) obtained from seeps for use in medication, waterproofing, and warfare several thousand years ago. The ancient Chinese frequently dug shallow pits or horizontal tunnels at the seep locations in order to recover the oil. In Baku, Azerbaijan, there are still gas and oil seeps that are permanently alight and have been used to light caravanserai since the times of Marco Polo and the Silk Road. With the dawning of the modern era in Oil Creek, Pennsylvania, Colonel Edwin Drake drilled the first well to intentionally look for oil in the subsurface in 1859. Again, this was based on direct identification of seeped hydrocarbons at the surface. Initially, the oil was used to provide kerosene for lamps, but the later invention of automobiles drove up demand and ushered in modern methods of oil exploration.

Around the turn of the century and up until the 1950s, the main exploration tool used for finding oil was the use of intensive and detailed geological mapping. This was frequently in terrain that was remote and inhospitable. The early pioneers working their way through the jungles of Burma, the deserts of Iraq, or the mountains of Iran would conduct detailed evaluations of the nature and distribution of rock units that could represent potential reservoirs, seals, and source units, as well as the frequency, orientation, and geological history of folds or faults that could act as traps for the migrating hydrocarbons. If all four of the features required for oil or gas to be created and trapped could be recognized in a region, then a variety of play concepts could be generated. Detailed local study might identify a suitable target (prospect), and then a shallow well would be drilled to test the features.

One of the most important recent discoveries in petroleum studies has been plate tectonics. Not only has plate tectonics revolutionized the earth sciences, but they also have provided a conceptual setting for oil exploration. The movement of plates around the surface of Earth creates large-scale depressions into which substantial quantities of sediments eroded from the surrounding high ground may accumulate. These accumulations can exceed thicknesses of several thousand kilometers and are referred to as sedimentary basins. By comparing basins around the world and by analogy to existing producing hydrocarbon regions, an exploration team can say which basins are worth looking at in more detail. Then the exploration team spends time ensuring that within such a basin there exist all the key elements that control the presence of hydrocarbons. Assuming that all the needed features are present, the team agrees that the basin contains a viable petroleum system, and prospect generation can proceed.

In modern exploration programs, the mapping of gravity and magnetic anomalies would normally be the first two methods to be applied to a new basin or region being evaluated. These techniques would be used to identify large-scale changes in the structure of the basement and sedimentary basins, and major differences in rock density, such as the influx of dense igneous rocks or light salt into a sedimentary sequence. These techniques are large scale, can be applied over both land and water, and can even be collected remotely from plane or satellite.

At the same time, remote sensing of onshore areas by large-scale photogeological surveys and, after the 1970s, by satellite imaging, can identify areas with anticlinal and faulted structural features, seeps, or salt domes frequently associated with oil occurrences. Offshore remote sensing of the sea surface can lead to the identification of slicks associated with the seepage of oil (both natural and manmade) into the water column. A coarse two-dimensional grid of seismic data is then collected to obtain a picture of the subsurface in the area to be targeted. Seismic data collection involves the generation of a seismic wave using an energy source such as an air-gun in water, dynamite in drill holes inland, or a truck with a plate that is thumped down onto the road/soil surface (vibroseis). The wave travels through the earth’s rock layers and reflects back off key surfaces. The time it takes for the waves to be received back at the surface along with the waves’ strength is recorded via geophones and displayed on a seismic section. Processing the two-dimensional seismic sections using highly sophisticated software reveals the detailed structure of the subsurface and in certain circumstances shows the presence of direct hydrocarbon indicators such as bright spots associated with gas/water differences. Primarily, though, seismic data are used to indicate the nature of folded and faulted structures that could prove to be suitable hydrocarbon traps. These structures are frequently referred to as leads.

The objective of seismic acquisition and processing is to acoustically image the subsurface in a geologically accurate manner with the highest resolution possible. For a detailed analysis of a small area representing a field or prospect, a high-density and calibrated three-dimensional seismic survey is performed and the data are collected. Modern technology also allows scientists to accurately map changes in fluid movements through time (repeat multiple 3-D seismic surveys, known as 4-D seismic survey). This technique is now particularly important in monitoring production performance of the reservoir.

Ultimately, however, the only way of confirming the presence or absence of hydrocarbons at depth is by drilling the prospect. In certain areas of the world where drilling is cheap and the subsurface has been explored extensively, such as certain onshore basins of the United States, drilling is commonly preferred to extensive and expensive seismic acquisition. Wells are then analyzed using electric, sonic, and radioactive logging techniques that measure characteristics of the rocks and fluids. These methods can identify the presence of oil and gas, which can then be tested to see if they occur at commercially viable production levels. On the other hand, at a cost of over 10 million dollars per offshore exploration well, the oil companies are likely to employ the sophisticated battery of direct and indirect detection techniques before resorting to drilling in these areas.