Contributions of applied geophysics to petroleum exploration are discussed. We succinctly look over geophysical prospecting in exploration of structural and stratigraphic plays. Structural prospecting has been the first, main concept for petroleum exploration through seismic imaging of subsurface. Stratigraphic prospecting was eventually successful as a by-product of structural exploration before digital seismics were introduced in 1960's through 1970's. Wireline logging provides us of basic data for our subsurface mapping and formation evaluation, and for calibration and improvement of quality of seismic processing. Geophysical prospecting was performed as; (1) gravity profiling in the latest 19th century, (2) single channel refraction and/or reflection profiling in late 1920's through 1930's, (3) multichannel reflection seismic survey to minimize noises by stacking CDP gathers during 1950's, (4) analysis of subsurface seismic velocity data qualitatively defining subsurface lithology and fluid in 1960's through 1970's, (5) analyses of allostratigraphy and sedimentary facies in 1970's, and (6) 3D seismic surveys that lead detailed, regional paleogeology and definition of stratigraphic plays in 1980's through 1990's. Seismic stratigraphy is a key in stratigraphic prospecting. We now can perform seismic geomorphology and reconstructive basin analysis based on 3D seismic data-cubes normally and inversely processed. 4D surveys represent changes of physical properties of subsurface formations through time. Future progress in the geophysical prospecting is expected in (1) combining of macro- and nano-surveys; that is, clarifying and solving enigmas in petroleum technology, e.g., primary migration of oil and gas, and efficient development of shale resources. Human development in technological education and management system of human resources are also the keys for this progress. Multidisciplinary education of geology, applied geophysics is desired for effective performance in petroleum exploration and exploitation. High seismic resolution is required for not only structural imaging but also rock physical one.
JOGMEC/MOECO carried out an onshore seismic survey with explosive sources in 2012 in Cambodia. The project was executed under the Basic Agreement for the Study and Survey Program in Block XVII onshore Cambodia between JOGMEC and CNPA. This was the first experience of seismic data acquisition with explosives in Cambodia. Therefore, there were a lot of issues to be resolved before the field work was launched. One of most serious issues was the implementation of safe operations in a Mine/UXO contaminated field. Other issues were a lack of regulations in Cambodia, education of non-experienced Cambodian people including both government staffs and local residents, explosive management and so on. Firstly JOGMEC/MOECO hired excellent Cambodian national staffs and further educated on seismic surveys. Then, they seriously and actively worked for communications with both government staffs and local residents. Such efforts resulted in good cooperation with all relevant parties even though unexpected procedures caused delays in the schedule. Finally, the seismic survey was successfully completed and no lost time for injury was recorded. A lot of knowledge on the seismic survey has been accumulated for all relevant parties through this experience. The procedures established by JOGMEC/MOECO through this survey will be one of the standards for onshore seismic data acquisitions in Cambodia.
Broadband acquisition and multi azimuth acquisition techniques in marine seismic survey are currently drawing considerable attention to improve the imaging for complex geology such as sub-basalt and sub-salt imaging. In the Block DWR, offshore Sabah, Malaysia, complex toe-thrust structures are existing and conventional legacy seismic data over these structures is suffering the deterioration of imaging due to complexity of geology and existence of shallow amplitude anomalies. JX Nippon Oil & Gas Exploration (Deepwater Sabah) conducted 3D seismic survey with combination of broadband acquisition technique using slant streamer and multi-azimuth (two azimuth) acquisition technique over the Block DWR. Broadband acquisition technique enhanced low frequency components in the seismic data and provided with improved imaging in toe-thrust structures and deeper seismic events. Processing for merged multi-azimuth seismic data is in progress, however, the seismic stack of each azimuth show better illumination than the other in different places. This fact indicates that merged azimuth seismic data can show better imaging and better seismic event continuities.
Since farming in to an offshore exploration block of Elf Gabon (presently Total Gabon) in 1974, MPDC Gabon Co., Ltd. (subsidiary of Mitsubishi Corporation) has been acting E&P business in Gabon. The Nguma permit is located in northern offshore Gabon approximately 65 km southwest from Port Gentil city and covers an area of 1,199 km2 (presently reduced to 600 km2). The turbidite sandstone in several geological periods is expected as possible reservoir in this area, and the main target is the Batanga sandstones in the Maastricht period. This facies is missing in the some wells and possibly to be distributed unevenly, and grasping Batanga sandstone distribution is one key point of this exploration. The distribution of reservoir sandstone was predicted using Extended Elastic Impedance (hereafter referred as EEI). EEI can approximate reflectivity or impedance to several elastic parameters (such as bulk modulus, Lamé parameter, rigidity modulus, and poison ratio) by a projection of acoustic impedance (AI) and gradient impedance (GI) data (AVO attributes). Gamma ray (GR) is the most suitable well log item to indicate sandstone reservoir and this prediction is first target for this study. However it is not tied to elastic modulus directly like as water saturation. Therefore it may not be necessarily derived from AVO attributes, and then the feasibility evaluation for applying EEI in this area was applied by taking empirical correlation. In data processing, EEI inversion with its proper (AVO friendly) pre-conditioning was applied on 3D seismic bin gather after pre-stack time migration. This EEI inversion derived pseudo GR and it has fine consistency with real GR at not only control well but also new well. In interpretation, turbidite sandstone distribution was predicted and delineated using EEI data.
Although sub-basalt exploration has been achieved great attention, the progress is very limited due to the difficulty of imaging beneath the basalt. Most of seismic energy are usually scattered and attenuated by thick basalts. Also Basalts usually generate a lot of multiples. As a result, quality of seismic data are usually not enough for interpretation. But recently, seismic techniques are advanced rapidly and some of those new techniques are expected to be effective in sub-basalt imaging. In this article, we review issues relating to sub-basalt imaging and some of the new recently advanced techniques, especially de-ghosting techniques in data acquisition are reviewed because low-frequency is a key factor in sub-basalt imaging.
As the deep water drilling technology continues to evolve, sub-salt basins has become one of the most prospective targets for hydrocarbon reserves. Even though sub-salt is a difficult target for seismic imaging, various advanced technologies have been developed in the lost decade and difficulties encountered in the new prospect have been reduced. New seismic acquisition technologies such as broadband seismic, full-azimuth and ultra-long offset acquisition mitigate the problems from the presence of salt. Sub-salt imaging techniques has also evolved with the aid of the advanced new acquisition technology, high oil price, and low cost high performance computing (HPC). Anisotropic RTM has became a usual technique in the Gulf of Mexico, and FWI grows in use and popularity nowadays.
A new seismic attribute method using seismic attenuation is present. Contrary to conventional seismic attributes providing reflection strength and phase properties, the attenuation attribute method maps relative Q spatially. Although the method has disadvantage in resolution, it potentially bears big advantage in applicability to analyses of physical properties of rocks in less reflective areas such as volcanic and highly-faulted areas where continuous reflections are rarely observed. The present paper reports on its applicability to evaluate fault seal ability, through a case study using 3D seismic data over two oil fields. As the result of the case study, a fault that traps oil is characterized by lower attenuation, whereas a fault that doesn't trap oil is marked by higher attenuation. According to recent laboratory measurements, seismic waves highly attenuate when they pass through a fault with lower coupling, reversely they don't attenuate very much within a fault with higher coupling. Assuming that the strength of fault coupling is proportional to the ability of fault seal based on existing microscopic observations on fault contact surface, we can form a hypothesis that the higher the fault seal is the lower the attenuation becomes. The hypothesis is supported by the present case study.
We apply full waveform inversion (FWI) to a land seismic dataset acquired along a long crooked survey line in a geologically complex mountainous region in Japan. The conventional reflection processing has been difficult due to the acquisition parameters and weak reflection signals at near-offset traces. However the dataset contains clear wide-angle reflections and refractions. FWI exploits these waveforms, and successfully yields a P-wave velocity model for the survey area. The FWI velocity model images the geological structure for the area including a large thrust structure. We validate the velocity model by comparing with the previously obtained migration image and the sonic logs. In addition, the synthetic waveforms generated from the FWI velocity model agree well with the observed waveforms, further confirming the validity of the FWI velocity model. Our results suggest that FWI is potentially a good velocity imaging technique for a geologically complex area.