Imaging and mapping technologies of the subsurface structure including 3D seismic methods have experienced great improvements in recent years. As exploration targets become more complex and exploitation requires more geological information, more accurate imaging and mapping of subsurface are necessary. In this paper, the author summarizes recent exploration targets and requirements in the exploitation stage, and then, reviews current status and future of subsurface imaging and mapping technologies. We should continue to make efforts toward more detailed and quantitative evaluation of subsurface.
Exploration in frontier areas, where the target structures are poorly constrained by seismic images of low quality, requires geologic models and their seismic expressions to interpret the geophysical data. This paper proposes a methodology to combine two modeling techniques, analogue experiment and seismic modeling. Analogue experiments can provide admissible geologic models that constructed from regional tectonic environment and rock physics, and seismic modeling can produce the seismic image of the models. The experiments performed on a horst structure buried under thrust-belt (Deep Horst in Thrust-belt : DHT) showed that the horst height has significant impact on the sequential development of the thrust-belt. This suggests that such exploration targets may be discovered by examining geometric anomalies within the thrust-belt. The seismic modeling produced stack and migration images of the structure that can be used as templates to explore possible horst structures under thrust-belts.
We analyzed azimuthal anisotropy of seismic velocity using 3D land seismic data in a Middle Eastern oil field, for predicting anisotropic horizontal stress and/or possible fractures. By applying sufficient correction, we extracted trustworthy distribution of azimuthal velocity variation from the 3D seismic data, which would represent distribution of the in-situ horizontal stress. The 3D data was acquired with a rectangular geometry, in which the inline maximum offset was 3.6km and the crossline maximum offset varied from 1.8 km to 2.2 km periodically. The nominal fold was 36 for a 25×25 meter bin. For using sufficient volume of data for individual azimuthal angles, we first generated super-gathers with 400m x 400m super-bins. To minimize the influence of structural dip on the super-bin that presupposes flat reflectors, we applied a dip correction, which do not harm the seismic anisotropy information of the data. Subsequently, we extracted azimuth dependence of seismic velocity at each super-bin. This analysis delivered 1) intensity of velocity anisotropy (the ratio of highest and lowest velocities) and 2) orientation of high velocity. These outputs were visualized by vectors at each super-bin, whose directions and lengths corresponded to the orientation and strength of velocity anisotropy. The result of velocity anisotropy presented that the highest velocity orientation was predominantly along NE-SW, which turned out to be consistent with the in-situ maximum horizontal stress orientation interpreted from borehole breakout of the resistivity image log data. Besides, the magnitude of seismic velocity anisotropy increased towards NE, i.e., to the proximity of active mountain range, which may indicate lateral variation of in-situ stress magnitude.
Methods used to reduce depth uncertainty and improve image quality for the reservoir section of the Ichthys Field, Browse Basin, offshore Australia, were examined. The methods implemented were detailed tomographic velocity modelling combined with 3D prestack depth migration (PSDM) and subsequent well calibration. Velocity analysis for 3D prestack time migration (PSTM) presented geologically implausible and undulating RMS velocity patterns at reservoir depths across the main section of the field and the reservoir depth accuracy of the result was low. Further investigation revealed that the velocity distortions were primarily induced by shallow Tertiary sequences, containing highly contrasting, narrow, elongate velocity anomalies. To solve these problems, Kirchhoff PSDM was applied. A hybrid velocity modelling method combining layer-based tomography with grid-based tomography was applied to obtain more accurate velocities. Firstly, layered/blocky modelling combined with dense residual moveout picking and 3D finite-offset tomography enabled the construction of a complex velocity model in the shallow section. Subsequent grid-based global tomography with constraints was then used for updating the entire velocity field. The finally derived velocity field was systematically correlated with that observed at the wells, and the corresponding depth structure produced by PSDM appeared to contain less distortion and to be more geologically realistic. The resultant velocity model is currently being incorporated into the structural evaluation and subsurface characterisation for the Ichthys Field.
This paper attempts to review the historical background, petroleum geological significance, major contributions and basic methodology of “seismic geomorphology”, which has recently been developed as an integration of 3D seismic technology/3D visualization technique and sedimentology/sequence stratigraphy. Seismic geomorphology may be defined as the study of sediment body morphology and properties using 3D seismic data, and is expected to be indispensable for future sedimentology and hydrocarbon exploration/development, since seismic geomorphology provides detailed information on the sediment morphology, distributions and depositional processes. From the sedimentological aspect, seismic geomorphology clarifies detailed internal structures of depositional elements, and improves general depositional models of various depositional systems. From the petroleum geological aspect, seismic geomorphology plays an important role to bridge over sedimentology, geophysics and reservoir engineering in an integrated reservoir characterization workflow. In a seismic geomorphological analysis procedure, basic sequence stratigraphic framework should be constructed, first, and reconnaissance survey over 3D seismic data should be conducted, next. Subsequently, targeted reservoir rock bodies are displayed in detail by slicing, horizon surface displaying, volume attribute analysis and trace shape (seismic facies) analysis on the 3D seismic data. Finally, sedimentological interpretation is conducted for the displayed sediment body to determine depositional elements, processes and facies distributions. It is regarded that proper pattern recognition and blend of spatial seismic geomorphology and temporal seismic stratigraphy are the keys to obtain effective solutions in the seismic geomorphological analysis.
Time-lapse three-dimensional (3D) seismic monitoring study was conducted in the JACOS Hangingstone steam assisted gravity drainage (SAGD) operation area, Alberta, Canada. The objective of the study was to delineate steam-affected areas using differences between two 3D seismic data acquired at different production stages for efficient reservoir management. The time-lapse surveys were acquired in February, 2002 and in March, 2006. As repeatability is important for the time-lapse seismic surveys, the two 3D seismic surveys were recorded with nearly identical field acquisition parameters and the data sets of both surveys were processed with identical processing flows. P-wave and S-wave velocities of oil sands core plugs from the field were also measured under various pressure and temperature conditions to understand the relationship between seismic velocities and reservoir conditions. The laboratory measurement results were combined and a rock physics model was proposed to predict velocity changes of the oil sands under reservoir conditions expected during SAGD operations. The two seismic volumes show significant differences in seismic character within the reservoir and time delays below the reservoir around the active SAGD well pairs. Synthetic seismic data based on the rock physics model were analyzed to evaluate seismic response changes of the time-lapse 3D seismic survey. From our analysis, the differences of the seismic responses between the two 3D seismic volumes can be quantitatively explained by P-wave velocity decrease of the oil sands layers due to the steam-injection. In addition, our result suggests that a larger area would be influenced by pressure than by temperature. In conclusion, the time-lapse 3D seismic monitoring along with the rock physics model is useful for qualitative and quantitative estimate of the rock property changes of the interwell reservoir sands in the field.
This paper introduces a workflow to compute the fluid saturation in a reservoir using 4D (time lapse) seismic data. In this approach, the decomposition of the impedance into velocities and density is the key to derive the quantitative reservoir properties using the currently available rock-physics applications. However, the decomposition is a challenging task due to the severe influence of noise on the analysis especially when the seismic data has a maximum-offset angle less than 30°. The proposed method of impedance decomposition simultaneously solves three elastic impedance data derived from the seismic inversion of angle stacks, where the most important reason that makes the decomposition difficult is the effect of noise. Theoretical investigation revealed that the offset-consistent component of noise mostly affects the determination of density but the noise effect is not severe to determine velocities. On the other hand, the effect of the random component of noise appears equally in the determination of velocities and density. Based on this result, the author developed a method of decomposition incorporating rock-physics bounds as the constraints for the analysis. Afterwards, a sequence of rock-physics theories is applied to transform the resultant properties into the bulk modulus and density of the in-situ fluid. Eventually, solving three simultaneous equations for the bulk modulus, density and the total saturation of three possible fluids; we can compute the saturation of water, oil and gas. As a case study, the workflow successfully visualized the change in fluid saturation based on an actual 4D seismic dataset of the Snorre oil field in the North Sea.
Recently the marine controlled-source electromagnetics (CSEM) is applied commercially to the problem of detecting the presence of hydrocarbon filled layers in the sub-sea formations, and a number of companies are now providing this service. In this paper, we return to fundamentals of electromagnetic (EM) method. Transmission frequencies used in the marine CSEM are typically between 0.01 and 10Hz. As such low frequencies, the behavior of EM field in subsurface is governed by the diffusion equation rather than the wave equation. The EM field radiated by an electric source can be considered to consist of two different modes : a TM mode component and a TE mode component. An analysis of both the TE and TM mode holds the potential to determine if the increase in the measured EM field is caused by the presence of hydrocarbon saturated reservoir or other resistive bodies in the subsurface. The physics of CSEM in shallow water is also examined by using simple 1-D models. This would show that the concept of air interaction, rather than air wave, is appropriate because of complex coupling between signals interacting with seafloor and air. We then apply the inversion analysis to frequency-domain CSEM sounding data acquired in the offshore of Australia, investigate how the marine CSEM could reconstruct resistivity image of gas bearing formation. The survey area comprises two towed lines, covering discovered gas field. The transmitter antenna is a horizontal electric dipole of length about 250m, towing along profiles at an average of 30m above the receivers. For the inversion process, we use a starting model with homogeneous background of 1.0 ohm-m below the sea bottom. No further a priori information is utilized during the inversion. The inversion reaches acceptable data misfit, produced images of the resistive target which can be interpreted gas bearing formation.
Research and development on geological disposal of high-level radioactive waste have been carried out by Japan Atomic Energy Agency (JAEA, previous JNC) in Horonobe, the northern Hokkaido. The geology of Horonobe comprises the Neogene siliceous marine sediments which contain saline water. The origin of saline water is one of the most fundamental factors for evaluating geological environments. The synthesis of geo-scientific understanding contributes to develop a performance assessment methodology by providing a basis for setting parameters and constructing conceptual model. Geological environments are also important for exploration of oil fields. This study analysed the various data of JAEA reseach for hydro-geochemistry of Horonobe. The squeezed water and the lifted water from eight boreholes drilled in Horonobe are analyzed on oxygen and hydrogen isotopes and Cl- concentration by JAEA. Based on the analytical data, the groundwater is subdivided into three groups derived from the mixtures of meteoric water and different saline waters, which distributions are consistent with the regional geologic situations. δ18O, δD and Cl- of pore water during diagenesis of the siliceous sediments are calculated based on the assumption that water/rock interaction makes redistribution of the isotopes in rocks and pore water during the phase transformations from opal A through opal CT to quartz in open systems, then that water in biogenic silica is expelled to pore-space during the successive compaction in closed systems. The measured data points fall approximately on the mixing lines between the meteoric water and the pore waters calculated on the assumption of the phase transformations. The geological study on the diagenesis of the field shows that the silica phase transformations actually occur near the depth calculated from the porosity reduction, indicating that the transformation and the succeeding compaction contribute to the origin of the saline water.
Advances in sampling and analytical instruments permit the isotopic analysis of very small amounts of hydrocarbons that are collected from the circulating mud stream during drilling. This new technique provides detailed isotope logging and is called mud gas isotope logging (MGIL;Ellis et al., 2003). MGIL provides useful information on petroleum exploration and production, such as origin of gas, thermal maturity, assessment of seal effectiveness, and identification of hydrocarbon-charged intervals.