Until the digital processing of seismic data was introduced to Japan in 1968, Japanese geologists had no means but to depend on the seismic depth profiles which displayed very short lines showing inclination of reflection planes picked up by seismologists. There was only a small discovery of the Tuchizaki-oki field those days. After the air-gun and digital processing became available in 1968, offshore exploration activities expanded and led to the discovery of the Aga-oki field in 1972. From the viewpoint of seismology, this discovery was based on stack sections and “hand-made depth sections” which displayed continuous curves indicating prominent reflections. Abnormal reflection patterns such as bright spots, flat spots and pull down began to attract geologists' attention in the mid-1970s, and various processing menus in addition to depth profiles were provided for geologists. In 1981, certain abnormal patterns were found in seismic record sections obtained in 1973, an exploratory well, Iwafune-oki SIM-1 was drilled at the center ofthe distribution area of such anomalies and became the discovery well of the Iwafune-oki field. Based on these experience, I emphasize the importance of stack sections in conjunction with wide and deep geological knowledge and thought. -Oil and gas fields are present in geologists' brains-
The Daleel oil field is located at the western corner of the Wadi Aswad contractarea of Japex Oman Ltd. Basic seismic exploration of 2, 883km in line length in this area wascarried out during 1981-1983. Several additional seismic surveys including two 3D surveys wereperformed later in the Daleel oil field and adjacent area for the purpose of reservoir delineation. Specifications of the former field data acquisition were basically 120 channel and 60 fold. Westarted first with end-on spread at 30m receiver and source intervals, and a few months laterchanged it to split spread at 50m receiver and source intervals. The latter specifications weresimilar to so-called ‘(ideal) stack array’ which was discussed later a lot on The Leading Edge ofGeophysics in 1986. This change seems to have contributed to speeding up work progress and toacquiring data in requisition of broad band, no much skew, and wide dynamic range, with leastnoise for tracing subtle and cache information. The stack array was generally good as spacedomain filter design and needed no much additional, irrationally large labor efforts. Uphole shooting from the depth of 40m on average at 465 points was useful not only forcalibrating weathering correction data, but also studying shallow layer situations, which did notalways fit typical two layer model. Interpretation pitfalls were checked occasionally on cross points of lines when trackingstratigraphical or lithological features in 2D seismic sections. After checking surface seismic velocity data with those of nearby wells we found veryreasonable harmony between them and successfully estimated a standard velocity function. Our main concern was to delineate reservoir features of the Daleel area. The effort delineatingthem also resulted in better definition of fault systems and subtle structural features. Everyinformation used here contributed to integrating complete interpretation.
This article introduces the gravity methods for petroleum explorations. Gravitational methods are based on the sensitive measurement at the surface of small variations in the gravitational field, but the measurement technology of land gravimeter has already established. In the present, the new trends of gravity measurement are coming. Gravity measurements by ship and airplane are possible. Therefore, Bouguer anomaly map of Japan map was compiled from about 350, 000 land gravity data, about 1, 000, 000 ship-borne gravity data and altimeter data from satellite in area where observation data are sparse, and the gravity basement of Japan was obtained from these data. New technology, which is capable of accurately measuring the full gravity gradient tensor, has only recently become available in the exploration geophysics domain from military research projects (e. g. the Bell Geospace gradiometer). Gravity gradiometry data have superior characteristics to traditional gravity measurements. The components of the gravity gradient tensor hold much more information about the subsurface than do traditional potential field measurements. Such gravity data can constrain geology in areas where seismic interpretation is difficult, such as sub-basalt and sub-salt. These data are capable of resolving shallow structures, such as the limbs of salt domes, far more accurately than can seismic surveys.
INPEX Corporation has performed six cases of onboard processing during seismic data acquisition. These were conducted in tight time schedules of contract or operations. Another necessity of the onboard processing comes from geologist's view; hoping to perform seismic acquisition and its data analyses as same as geological surveys in front of outcrops, cuttings samples, wireline logs, etc., to look at subsurface to locate a prospect to drill timely, and to carry out simultaneous corrections and additions of survey routes and samples. The first case was in 1994 for North Sumatra, Indonesia. It aimed at performing 2D seismic data quality control and quick-look examination of tentative location of the second wildcat drilling, which was urgently planned as a back-to-back well with the first one. The latest one was in 2001 for Masela PSC Block, West Arafura Sea, Indonesia. The onboard processing was performed to depict 3D image of First Track Cube (700km∧2) covering the area of Abadi-1 gas discovery. INPEX was successful in this partial onboard seismic processing, and in timely depicting the main structural feature of the gas discovery. It was very much helpful in fixing the appraisal drilling location in a tight time schedule. The tight schedule was due to contractual (e. g., short period to decide PSC transfer from the first three years' period to the fourth year) and other operational constraints (e. g., limited reliable man-power). Further progress in computer processing speed is desired to simultaneously analyze acquired seismic data onboard, and to optimize exploration and development schemes by adding or revising the survey lines and cubes. Contractual environment for seismic service-contractor should be well prepared in order not to stress them to perform their services too quickly but to well prepare, tune up all equipment and man-power for their services and be on site in time to start the survey. Too severe penalty clause could cause quick but bad services.
A two-dimensional high-resolution seismic survey was conducted in 2001 for exploration of methane hydrate in the eastern part of Nankai Trough area, where a huge amount of methane gas hydrate has been suggested by many researchers. Seismic survey lines were settled with coverage of four types of BSR (Bottom Simulating Reflector) distribution, which is a significant indicator of free gas existed below the methane hydrate stability zone. Interval of each seismic survey line was approximately 5km, and total survey length was 2, 800km. Tuned air gun array with total volume of 1, 060 cubic inch (17.37 liters) was used as an energy source, and two types of field data were acquired through whole survey lines using two streamer cables towed with water depths of 3m and 6m. High continuity of seismic events could be obtained by usage of the depth of 6m, which is a quite conventional method, but resolution of seismic events is not so high especially at BSR with this depth. Using the towing depth of 3m, a high-resolution seismic data was acquired along whole survey lines. Combination of these types of seismic data including deep-tow seismics with well log data to be obtained in 2003 is expected as an effective geophysical indicator for exploration of methane hydrate and free gas in the Nankai Trough area.
In many cases, predictions of turbidite reservoir distributions are not simple because their depositional patterns and property of reservoirs are liable to variation. In this case study of Iwafune Oki oil and gas field, integrated approaches for the prediction of the turbidite sand reservoir distributions are reported. The rock facies and property at the well were correlated to the seismic data to clarify causes of the seismic amplitude. It is obvious that the gas sands showed low acoustic impedance (AI) and their AI contrasts generated high amplitude reflections. For the oil-producing interval, we found that the amplitude was caused by AI contrast between different rock facies. The acoustic impedance data inverted from the 3D seismic traces were used for the reservoir interpretation. The gas sand reservoir could be predicted directly on as a low AI zone. For the oil reservoir, the net sand thickness distributions were estimated with a relationship between the net versus gross thickness in the lower AI intervals derived at well. After those evaluations, we drilled two wells, the northern case expected as oil reserve and the southern one expected as a gas reserve. The recent drilling results reveals that, in northern one, for the extent of existing reservoir, our estimation was almost correct, but the main target expected as new reserve, was over pressured shale. For southern case, we drilled a high amplitude portion in existing reservoir expecting as thick gas zone, was thick and low impedance porous sandstone but an oil reserve. These lessons show that the estimation of the target impedance was success, but the interpretation of impedance was non-unique. We should recognize such a risk whenever we use seismic attributes for E & P studies.
Reservoir structural study was conducted in the offshore Tshiala oil field, Democratic Republic of Congo. In reservoir characterization, accurate structural interpretation is crucial as the reservoir structure is treated as one of hard data. In order to construct high-frequency reservoir model from seismic data, new method for the improvement of seismic resolution was developed. This method, called pseudo-seismic processing, consists of the following steps. High-frequency reflectivity series are derived through the combination of two deconvolution schemes, and then pseudo-seismic traces are generated by convolving an objective wavelet. This procedure enables the reliable correlation of seismic data with the detailed reservoir structure at the well locations, and high-frequency 3D reservoir structure can be interpreted. Constructed model using this method together with geostatistics was proved to be useful as the excellent history matching was achieved in reservoir simulation.
The purpose of this study is to verify the applicability of the 3D seismic inversiondata for the hydrocarbon determination. The 3D seismic data consisting of two angle stack data (0-22 .5 degree and 22.5-45 degree), were inverted into acoustic impedance and Poisson's ratio data. The target interval is the Upper Cretaceous, Coniacian sand and we tried to discriminate between the gas sand and the water sand by them. The two sets of angle stack data were inverted for angle impedance. Angle impedance is a physical quantity which depends on acoustic impedance, shear impedance and the angle of reflection wave. The match between the angle impedance results from the 3D seismic data and the angle impedance logs is rated from good to very good with respect to both reflector placement and absolute angle impedance value. The near angle stack data results in the better solution than the far angle stack data. Poisson's ratio and acoustic impedance volumes were calculated from the angle impedance inversion results. Comparison of the calculated AVO attributes with the well log data shows that the acoustic impedance volume is of very good quality and tha Poisson's ratio volume is of good to poor quality. A lithology cube was produced on the basis of lithology probability calculated from the cross-plotting of Poisson's ratio versus acoustic impedance. Although an amplitude anomaly of full stack data yields only a sand distribution, a gas sand distribution was interpreted by using this cube.
A quasi-linear inversion method is a new algorithm for the 3D MT inversion that is going to stand the practical use in near future. The performance of this inversion method is quite high to realize the much shorter processing time than the conventional MT inversion method. In this paper, we have tested the quasi-linear inversion method by means of the numerical simulation. We applied the new inversion technique to the synthetic MT responses to the numerical models, and the input models were reconstructed successfully. We also applied the method to the real field data acquired in Minami-Noshiro area in Akita prefecture. The major geological features in this area are N-S trending structures of anticlines, synclines and faults. The 3D inversion results were consistent with the geological information. To visualize the highly complicated 3D structures of this area, the bird's-eye view was presented for the resistivity boundary at the Onnagawa formation top that was determined from the induction logging data. The geological structures of Nakazawa syncline, Sotooka anticline and the structural high of Minami-Noshiro oil field were clearly seen on the bird's-eye view. The major structure of the Noshiro thrust faults was also recognized in the western part of the bird's-eye view.
Reservoir monitoring is the key issue to design the optimum plan for recovery enhancements of hydrocarbon production. The Technology Research Center of Japan National Oil Corporation has conducted extensive field measurements to validate the current cross-hole technologies at the Lost Hills Oil Field, California in the first quarter of 2000. The experiments were composed of cross-hole electromagnetic surveys between fiberglass cased boreholes in both frequency domain and time domain methods together with seismic tomography. The time domain electromagnetic survey was designed to test the newly developed time domain down-hole transducer. However, the tool was broken down during the measurement. For the frequency domain electromagnetic method, we conducted three kinds of experiments, which are time lapse analysis with previously measured data, multi-frequency survey for higher resolution analysis, and measurement with setting up transducer in a steel cased hole. Multi-stage multi-disk borehole seismic source, which has been developed in collaboration between JNOC and OYO corporation was employed for seismic tomography. The subsurface velocity profile from seismic tomog raphy was utilized to calculate the formation porosity. Since the standard calculation procedure of sonic porosity is not adequate to apply for the diatomite reservoir because of its nature of high porosity despite very low permeability, a correction function was developed and employed. Then the resistivity profile was incorporated into estimation of fluid saturation in the reservoir by Archie's equation. The resultant values of porosity and water saturation are quite reasonable in comparison with reported ranges for this field. Assuming that the regional compaction being related to reduction of reservoir porosity has not been significant from 1997 to 2000 based on the ground subsidence in the area, we calculated a fluid saturation profile since the beginning of water flooding. We revealed the manners of fluid saturation change in the reservoir during water flooding through the time lapse analysis.
The advancement of geophysical technology has been based on the development of data acquisition system, computer technology, and data processing/analysis algorithms. The role of geophysics in the upstream division of oil and gas industry has been expanding. Geophysics is now used not only for exploration but also for exploitation, development, and production monitoring. Reservoir monitoring is a technology to monitor the movement of hydrocarbon through production. A typical technique to detect the hydrocarbon movement is comparing two 3 D seismic volumes acquired with certain time interval. Geophysical techniques for monitoring require very high accuracy in all aspects. Instrumented Oil Field is an advanced form of monitoring and its purpose is real-time reservoir management. It consists of permanently deployed sensors and so called smart well with remotely-controllable choke and valve for each reservoir. Results of geophysical data analysis are utilized to control the flow/injection rate to optimize the production. These technologies are so far monopolized by major oil companies, but they will probably be available and feasible for small fields of non-major oil companies in the future. Geophysical techniques to meet the severe requirements for monitoring will contribute to the improvement of geophysical applications to other stages such as exploration stage.
The phenomena in an oil reservoir during the super-wet combustion were experimentally investigated and evaluation of the super-wet combustion as an EOR from a technical point of view was carried out. First of all, the formation of stable wet oxidation reaction zone was experimentally confirmed in a combustion tube, when the ratio of oxygen and water injection rates were appropriately set. The result shows that two different types of flooding mechanism exists and oxygen/water injection ratio dominates the phenomena. That is to say, when much oxygen is supplied, the temperature goes up to the saturation temperature of water and generates steam which easily flows in a gas phase. In this case, steam flood may be the dominant mechanism to displace oil. On the other hand, the displacement of oil by hot water may be the major mechanism, if oxygen supply is not sufficient to generate much steam. Secondly, from the technical viewpoint, feasibility of both mechanisms for EOR was discussed. As a result, in the case of both types of flood, steam and hot water, it was found that the incremental oil recovery was 15-30 percent of the original amount of oil in the reservoir. This result shows that the super-wet combustion is a possible EOR technique.
We examined control factors of regional variation of gas-water ratio and iodine concentration based on the integrated evaluation of the subsurface submarine-fan distribution of Umegase and Otadai formations, the location and displacement of N-S trending faults developed in the Kujukuri District along the Pacific Coast of Chiba Prefecture, central Japan. The main results are as follows; 1) Umegase and Otadai formations which consist of turbidite successions as main reservoirs are the thickest nearly at the southwestern margin in the study area and have elongated northeastward as far as about 20 kilometers, i.e., the thickness of the formations decrease gradually northeastward, are considered as ancient middle to lower fan deposits formed under channel-lobe system based on the electric logging patterns. 2) Regionally, gas-water ratio increases as well as Umegase and Otadai formations thicken, and the distribution of gas reservoirs with high gas-water ratio, usually known as Mobara-type gas reservoir, are well coincident with that of submarine-fan deposits. Locally, extremely high gas-water ratio tends to be located in the west side of main eastward-dipping normal faults. These facts strongly support the idea that biogenic gas moved the southeastward, i. e., to the depocenter of the submarine-fan deposits and to the up-dip direction, and then it was trapped in the west side of major N-S trending normal faults. 3) The regional change of iodine concentration (I/Cl) is basically concordant with the thickness of the submarine-fan deposits. Iodine concentration tends to be highest near the depocenter of submarine-fan or turbidite deposits in open sea environments. This fact suggests that organic matters both of continental origin such as plant fragments in turbidites as absorption materials and of marine origin such as planktons as original iodine supplier are important to produce high iodine concentration in brine.