Field experiments for deep seismic profiling were conducted at Hidaka, Hokkaido and in the Takada Plain, Niigata Prefecture, to examine deeper reflection records in a geologic complex structure and in noisy circumstances mainly caused by human activities. The result from the Hidaka Line shows that diversity stacking suppresses bursy noises like trafic noise although energy by vibrator sources is not enough to penetrate deeper part. The experiment in the Takada Plain shows that a large volume of explosives is one of the most powerful and efficient sources and may give a seismic image of deep crust. Preliminary interpretation reveals complex structures in some deep crust at both experiment areas. Existing geophysical data such as gravity data strongly assist reconstruction and interpretation of a seismic section.
We present a comparison between the 2-D resistivity structural model (EM model) and the structural model derived from the seismic survey (seismic model) in the northern slope of Mt. Chokai. From 1989 to 1991, three types of electromagnetic survey and two types of resistivity survey were conducted along the seismic survey line that was acquired by JNOC in 1985. EM model was constructed from electromagnetic and resistivity survey data by using some inversion and forward modeling techniques. The well data (MITI Nikaho) and another electromagnetic data collected near by the well were used to the geological interpretation. Seismic model was derived by using inverse ray method (May and Covey, 1981). In this method, 2-D depth model is computed from reflection times of interpreted horizons on CDP-stacked section and spatial velocity function. As a result of the comparison, these models are not consistent in the deeper part, while they agree quite well in the shallower part. The uniqueness of the 2-D resistivity structural model was examined through some model calculations. Other possible explanations concerning the difference of these models are also discussed in this paper.
Talbot oilfield was discovered by AC/P12 joint ventures in December 1989. The AC/P12 permit was gazetted for competitive bidding and awarded in February 1989. Work obligation of the permit was composed of the seismic program involving acquistion of 2, 660km of 2-D and 3, 350km of 3-D data, seismic reprocessing and drilling of four wildcat wells. 3-D seismic acquisition covered the C2 prospect, which was delineated as a significant fault-dependent structural high during the bidding study in 1988. Interpretation of this oil field, subsequently named Talbot, revealed tectonic and stratigraphic similarities to the Challis oil field, 40km to the north. The Talbot-1 produced oil and gas at and unstabilized rate of 4, 981 BOPD and 3.71 MMCFGD in December 1989. The Talbot-2 appraisal also produced oil and gas at a maximum unstabilized rate of 4, 992 BOPD and 2.93 MMCFGD in June 1990. By comparison with Challis oil field, the reservoir sandstone of the Talbot oil field was expected to be thick and of excellent quality. However, it was composed of a thin bedded poor reservoir quality sandstone, alternated with siltstone, claystone and dolomite. The purpose of this study was to clarify the structure of Base Cretaceous horizon and upper Triassic reservoir itself together with the oil and gas reserves estimation of the Talbot oil field in detail. The study has clarified the structure of the Talbot oil field including the dip and minor faults of the reservoir section. The 3-D seismic interpretation was one of the most useful methods to evaluate a thin bedded reservoir like the Talbot oil field in detail.
A detailed delineation of structural configuration for the 3DA structure, which is located in the Gulf of Moattama, offshore Myanmar, was required for the current evaluation work. For this study, 650 kilometers of 2-D seismic data covering the structure were reprocessed and acoustic velocity measurements of the well core samples were made. The reprocessing and reinterpretation provided more reliable structural map and porosity distribution map of the reservoir of the 3DA structure. The structure composed of a carbonate build up exhibits a four-way closure. AVO analysis was applied to define a productive area from non-productive area of the structure. Also, seismic interval velocities were used to produce an average porosity map of the gas-bearing carbonate reservoir of the structure. The results made it possible to obtain more reliable estimation of the rock volume and the porosity distribution for the 3DA structure.
Quantitative geophysical study of seismic stratigraphy technique was examined through a case study of the prediction of sandstone reservoir distribution on the Minenobu structure in the Ishikari Plains, Hokkaido. This geophysical study was performed by the integrated interpretation consisting of seismic modeling, VSP, model-based inversion and conventional 2D seismic sections. As the result of this study, the distributions of two target sandstones are clearly delineated as described below. 1100m-layer sandstone reservoir in Atsuta Formation is distributed covering Minenobu structure with the width of 1km-1.5km, trending north-south direction and extending irregularly and discontinuously in east to west directions. This sandstone is estimated to deposit along the turbidity current with north to south direction in consideration of well data information. 1600m-layer sandstone reservoir in Ponsubetsu Formation is distributed only in the limited area with the width of about 500m almost along the line 84 V-1 on the higher part of the acoustic basement, trending north west to south east direction. Paleo depositional environment of this sandstone is estimated to be bar sand or channel sand in shallow waters according to the dipmeter and geological information. Model-based inversion is rather reliable and powerful technique for the lithological interpretation on the condition that original seismic data quality is good. VSP in a deviated well is useful technique for the lateral prediction of reservoirs. Recently the seismic stratigraphy technique has been remarkably advanced. Seismic stratigraphy has become a fully integrated working discipline, integrating the purely geological and geophysical approaches to exploration.
Predicting the reservoir rock properties such as porosity and clay content is essential for estimating reserves and planning the optimum development of oil and gas fields. Modern seismic data, especially 3-D seismic data, has played an important role in reservoir characterization. A study has been performed to predict the reservoir disrtibution and the rock properties using 3-D seismic data in the Amarume oil field. The reservoirs are mainly in Kitamata Formation of Miocene which is considered to be deep-sea sediment gravity flow deposits. Several sandstone layers, which are discontinuous in horizontal direction, bear hydrocarbons. Productive sandstone reservoirs lie approximately 900m below the surface and vary from 5m to 20m in thickness. 3-D seismic survey area is about 2km in East-West direction and 1km in North-South direction. Acoustic impedance was derived from the 3-D data volume by seismic inversion method. Such impedance data can be related to rock properties and physical conditions in the reservoirs. The relationship between acoustic impedance (velocity) and rock properties such as porosity and clay content was analyzed in the reservoir interval based on the results of well log data analysis. An empirical equation was obtained by linear regression analysis to effective porosity versus acoustic impedance with a correlation coefficient of 0.87. The correlation was improved by addition of a clay, shale or tuff content term in the linear regression analysis. These relationships indicate that an increase in clay or shale content rises P-wave velocity. This observation differs from that quoted by several other investigators. Seismically derived acoustic impedance (I) was converted to effective porosity (φ) using the equation [I=5.72-8.15φ]. Depth structures, effective porosity and effective porosity-thickness of the reservoirs were mapped based on the converted effective porosity. The results were consistent with well data. 3-D seismic data can be a powerful tool for predicting the areal distribution of reservoir rock properties.