Seismic Hydrocarbon Exploration - Literature review Example

Seismic Hydrocarbon Exploration Introduction Hydrocarbon exploration, also known as gas or oil exploration refers to the search undertaken by geophysicists and petroleum geologists for deposits of hydrocarbon occurring beneath the surface of the earth including natural gas and oil. The maiden scientific exploration for hydrocarbons was conducted in Oklahoma’s Cushing Field in 1912 (Reynolds 2011). Though the process remains the same, modern technology has seen some transformation in the involved processes. With the continued depletion of the reservoirs, the petroleum industry explores even deeper and in even more challenging environments using three common methods of exploration of hydrocarbons which include geological, geophysical and drilling methods. Geological studies on a prospective reservoir area call for knowledge on the region’s tectonics and geology in establishment of traps or structures, source rocks, seal rocks or reservoir rocks. The geophysical methods have been cited by Reynolds (2011) as the most common methods and encompass seismic, gravity and magnetic surveys. Finally, the minimally used geochemical surveys method of exploration involves collection of soil samples from the suspect area followed by laboratory testing for bacteria-formed microgas. The presence of non-bacterial gases would also be tested to identify the existence of abnormally high level concentration of gases which would call for further evaluation of seismic data. These methods would be applied in both offshore and onshore situations. Of importance in this paper though would be the seismic method of hydrocarbon exploration which as edited by Davies et al. (2007) has been the cornerstone of hydrocarbon exploration. This method rides on the Physics of the earth principles with the use of seismic energy to probe below the earth’s surface basically as an aid in exploring for presence of hydrocarbon deposits. Near or at the surface of the earth, this method would be used to measure the elastic features of the subsurface and identify any variations in these features, with variations pointing out at changes in lithology or pore fluids. This method could also be used for other purposes, notably archaeology, engineering and scientific studies. Just like in any other hydrocarbon exploration process, seismic exploration for hydrocarbon-bearing rocks begins with desk studies to review and identify viable sedimentary basins (Jahn, Cook & Graham 2008). Aerial photography could be employed to give information on formation of promising landscapes like anticlines and faults. For more detailed information, a field geological assessment would be employed followed by seismic survey. The seismic survey uses varied sound waves reflective properties on various rock strata where a source of energy would transmit acoustic energy pulse into the ground which then travels in form of synthetically generated seismic waves into the earth. With each varied geological strata encountered, some of the energy would be transmitted deeper down the layers of the earth while the rest would be reflected back to the surface. After the waves pass through or get reflected off the area of interest, referred to as the target by Royale Energy Inc. (2012), there would be cables and sensitive receivers referred to as hydrophones in water or seismometers or geophones on land that would then transmit electrical signals, amplify them, filter, digitize and record on magnetic tapes. This survey involves generating numerous seismic shots or source events from various locations in the target. The receivers would generate trace or seismogram representing the movement of the earth at the receiver with each trace having a reference time zero that corresponds to the time of the source event. The traces for each shot recorded on such medium as the magnetic tapes would be utilized in subsequent studies including processing, displaying and interpreting. The occurrence of an explosion or earthquake causes shock waves referred to as seismic waves to travel through the ground which in turn get reflected off the rocks in the subsurface just as ripples in a water body would be reflected off boats in water. Since boundaries between various rocks would often reflect seismic waves differently, such waves would be used by geophysicists to generate pictures showing the features of the subsurface rocks. According to Bayode, Adewunmi and Odunwole (2011), dynamite used to be a common source of energy but due to environmental considerations that favour lower energy sources like vibroseis and air gun, the latter have been adopted widely. The dynamite method would be more preferable in areas that require preservation of vegetation cover. With the availability of this data, data processing would follow suit usually done in vast computer centres with the aim of producing subsurface profile resembling the geologic cross section. The plotting could be done using a time scale or depth with the interpretation having two major components, stratigraphic and structural (Reynolds 2011). While structural interpretation of the seismic data would involve mapping the geologic relief of the varied subsurface strata using seismic data and information from outcrops and boreholes, stratigraphic interpretation considers attributes in a common stratum and the changes observed would be used to infer varied conditions of the reservoir like porosity, lithology and fluid content. Two forms of data processing could be used with the difference between these 2-dimensional, 2-D and 3-dimensional, 3-D being in the quantity of variations from which meaningful information could be calibrated (eds. Davies et al. 2007). As with the historical 2-D, the pictures of the cross section would be provided with the geophones along the fault lines or lines of vibration. It assumes the reflection points occur between the traverse and vertical lines. On the other hand, geophones in 3-D would cover a grid as opposed to a line by placing the receivers and sources in an aerial pattern. With the increase in data points, geophysicists could map a cube which yields a 3-dimensional image. In spite of the high costs involved with 3-D, most hydrocarbon exploration companies prefer the method as it minimizes resources spent concentrating on unlikely prospects. In fact, Royale Energy Inc. (2012) cites that over 80% of seismic explorations would be 3-D and Bayode, Adewunmi and Odunwole (2011) note that over 50% of seismic activity in the North Sea and Gulf of Mexico would be done in 3-D. It gives more accurate and detailed subsurface images, offering significantly higher qualities for signals than the 2-D approach. If the seismic exploration presents a promising geological structure, the presence and thickness of hydrocarbons and the reservoir’s internal pressure would be confirmed by drilling exploratory boreholes with Department of Mines and Energy (2008) referring to such wells drilled in search of hydrocarbons as exploration wells, popularly known as wildcats among the drillers. The length of time taken to drill a wildcat would depend on the geological conditions and depth with which the hydrocarbon occurs. The finding of a hydrocarbon formation would lead to initial well tests to determine formation pressure and flow rates. Lack of commercial quantities of hydrocarbon would lead to decommissioning and restoration to original condition while discovery of commercially viable quantities of hydrocarbon could lead to installation of a wellhead valve assembly. With the successful appraisal of the exploratory drilling, more wells would be drilled to ascertain the extent of the field with the wells drilled for the quantification of hydrocarbon reserves referred to as appraisal or outstep wells (Bayode, Adewunmi & Odunwole 2011). The establishment of the hydrocarbon field paves way for drilling of production or development wells. After their commercial lives, typically between 20 and 40 years, the onshore production installations get decommissioned and the area gets rehabilitated. Conclusion The seismic hydrocarbon exploration method occurs as the most common of all the hydrocarbon exploration methods. The process involves use of artificially generated sound or seismic waves released into the subsurface with the reflected and penetrating seismic waves informing on the availability of hydrocarbon reservoirs. Desk studies revealing the possibility of hydrocarbon reservoirs in the subsurface would pave way for seismic surveys including data interpretation with further prospect leading to drilling of exploratory boreholes and further production wells. The end of its lifetime would be marked by decommissioning and rehabilitation. References Bayode, OJA, Adewunmi, EA & Odunwole, S 2011, ‘Environmental implications of oil exploration and exploitation in the coastal region of Ondo State, Nigeria: a regional planning appraisal’, Journal of Geography and Regional Planning, vol. 4, no. 3, pp. 110 – 121. Davies, RJ, Posamentier, HW, Wood, LJ & Cartwright, JA (eds.) 2007, Seismic geomorphology: applications to hydrocarbon exploration and production, MPG Books Ltd, Bodmin, UK. Department of Mines and Energy, Queensland Government 2008, Petroleum and gas exploration. City East QLD, viewed 1 January 2013, Jahn, F, Cook, M & Graham, M 2008, Hydrocarbon exploration and production, 2nd edn, Elsevier, Oxford, OX. Reynolds, JM 2011, An introduction to applied and environmental geophysics, 2nd edn, John Wiley & Sons, New York, NY. Royale Energy Inc. 2012, 3D Seismic, viewed 1 January 2013, Read More