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Hydrocarbon Exploration Notes This brief collection of notes was compiled to answer a number of email enquiries requesting information about the use of hydrocarbon leakage indicators to explore for oil and gas. I have also included some basic information about oil exploration.
Hydrocarbon Seepage Detection and Mapping. “A look at the exploration history of the important oil areas of the world proves conclusively that oil and gas seeps gave the first clues to most oil producing regions. Many great oil fields are the direct result of seepage drilling.” Link 1952. “We usually find oil in new places with old ideas. Sometimes, also, we find oil in an old place with a new idea, but we seldom find much oil in an old place with an old idea.” Parke Dickey, 1958. Natural petroleum and gas seepage is an indicator of accumulations, migration pathways or mature source kitchens. Historically, onshore seepage mapping has proved valuable in many parts of the world including North America, the Middle East (Iran and Iraq) and Indonesia. Seepage mapping is much more difficult offshore but has been documented in the North Sea, Bass Strait and the Gulf of Mexico. A number of techniques have been developed for mapping offshore seepage including: SAR (synthetic aperture radar): a satellite mounted radar is used to detect the smoothing effect of a natural oil slick on the ocean surface. ALF (airborne laser fluorosensor): an aircraft mounted ultra-violet laser is pulsed at 50Hz onto the sea surface and any fluorescence caused by a thin oil film is detected\ and recorded. Hyperspectral scanner: an aircraft mounted detector records an image of the sea surface for a selected range of light wavelengths, which are selected to highlight any oil slicks. Interpretation of hydrocarbon leakage effects on conventional reflection
seismic data: commonly observed leakage induced features include: Accumulated hydrocarbons can occasionally leak from the trap by partial failure of the top seal or, more commonly, by a temporary breach of the seal during fault movement (an earthquake). Most seepage mapping techniques provide only a snapshot of present day seepage. The SAR technique requires multiple-coverage at different times to increase the chance of observing a seep, which may only occur episodically. Analysis of seismic data can however reveal past seepage phases. Seeped oil samples can be obtained by seabed coring or, in some cases, sampling a surface slick. Some indication of the quality of the reservoired oil can be obtained from these. The samples may have been significantly altered by biodegradation, evaporation or dissolving of more soluble components in the water.
Basics of Oil Exploration A commercial oil or gas field is formed when “source rocks” are buried at sufficient temperature and pressure to generate a volume of hydrocarbons that accumulates in a “trap”. The hydrocarbons move, or “migrate”, from the source to the trap along permeable pathways, predominantly uphill under the effect of buoyancy. The trap is formed by a permeable and porous rock, usually sandstone, overlain by an impermeable layer, usually shale. The sandstone/shale interface must form an inverted basin to accumulate hydrocarbons, preventing them from continuing to rise to the surface. Traps are commonly formed by tilted fault blocks, where a basin as been stretched by tectonic forces that cause it to break into a series of fault blocks. Most of the world’s sedimentary basins do not have sufficiently good source rocks to generate enough hydrocarbons to form commercial accumulations. When exploring in an unproven region, it is most important to reduce the risk of insufficient source. If there is no source, there can be no accumulation. Reflection seismic imaging of the subsurface is the main surveying tool used by exploration companies. After the acquired data is processed, images of the subsurface geology are produced. These images are interpreted to produce maps of the subsurface layers, which are used to identify potential hydrocarbon traps. Two types of reflection seismic surveys are used for hydrocarbon exploration. Separate seismic lines spaced from several hundred metres to several kilometers are used to provide an indication of the subsurface structure at relatively low cost. These are called 2D seismic lines. When the lines are spaced at 50m or less, continuous coverage, or 3D seismic data is obtained. This data is processed to produce a uniformly sampled 3D volume of data, which produces a much more accurate image of the subsurface geology. Some companies incorporate data from other survey types to refine their geological model. These surveys can include gravity, magnetic and various seepage detection surveys mentioned above.
Hydrocarbon Exploration Considerations Exploring for hydrocarbons is a risky and costly business, but the rewards can be large. Important factor to consider include: Are we looking for oil or gas? Developing a gas field is much harder than an oil field. A market for the gas must be found and a lot of production, processing plant and pipelines must be installed. What is the water depth (for offshore exploration)? Drilling and producing oil in 100m water depth or less is relatively easy, but the easy acreage has usually been explored hard. There are more prospects available in deep-water acreage (1000+m) but deep pockets are needed to pay the costs. A modern dynamically-positioned drillship can cost nearly half a million dollars per day to run. A relatively quick 30 day well can cost US$15 million to drill. What are the production terms? The government take varies considerably from country to country. Economic oil reserves for a shallow-water field can range from less than 10 million barrels to 40 million or more in a higher taxing country.
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