Diatomaceous rocks are widely distributed around the circum-Pacific region as Miocene to Lower Pleistocene deposits, including the Monterey Formation, California, USA (Ingle, 1981; Isaacs, 1987). The Middle to Upper Miocene Onnagawa Formation is a typical diatomaceous rock in northern Japan. The alternation of hard and soft layers is a characteristic feature, as shown in this picture. The hard layers consist of thick (< several tens of centimeters) porcelanite, while the soft layers consist of thin (< 10 cm) siliceous mudstone. The Onnagawa Formation is known as an excellent petroleum source rock in Japan. The total organic carbon (TOC) contents of porcelanite and siliceous mudstone are generally 0.1 to 3wt% and 2 to 6wt%, respectively. The Yurihara–Ayukawa oil field is located about 5 km west of this outcrop. The Onnagawa Formation is also a reservoir rock, in which oil and gas are reservoired in pores expanded by the diagenetic phase transformation of silica minerals. Moreover, the Onnagawa Formation draws attention as a shale oil resource. (Photograph: Yuichi SAITO, Explanation: Takashi TSUJI, Amane WASEDA and Satoru YOKOI; Photographed on May 22, 1988) The Monterey Formation is the primary source rock for oil in California. The light brown layers with fractures are Opal-CT chert. Because Opal-CT is still preserved in this rock, the rock was never heated to a high enough temperature to transform it into Quartz. Consequently, this outcrop probably did not reach the thermal maturity for oil generation. It is likely oil in fractures was generated at higher maturity areas in the basin center, and migrated updip into this lower-maturity fractured Opal-CT reservoir. Monterey shale has been producing oil for more than 100 years. (Photograph & Explanation: Kazuyoshi HOSHI; Photographed on March, 1992)
Oil and gas column heights and accumulation status within the reservoir can be understood from the relationship between top seal capacity and height of trap (level of spill point). In previous studies, such evaluations were limited to hydrostatic conditions. This paper presents a study on abnormally high pressure conditions where pressure increases stepwise in sandstone layers toward the core of a folded structure, as in the case study offshore Sabah, Malaysia. First, the stepwise convex-upward shaped oil water contact (OWC) is investigated in the field, from which it is concluded that abnormally high pressures caused the conditions. A gradually progressive pressure increase toward the core axis within the folded structure contributed to the reduction in the column height of oil accumulated at the central part, showing a stepwise upwardly convex OWC. Second, an attempt is made to quantitatively predict conditions by applying data from only the exploration well. Estimated oil reserves based on this form of accumulation were found to be more than double that assumed by the conventional flat OWC. This concept will widen the scope of applications for future oil exploration.
The purpose of this study is to estimate the average flow rate of subsurface formation water through an overpressured reservoir in a significant gas field. Oil (or gas)/water contact in a field can be recognized as a representation of the hydraulic potential surface. Gradient of the potential surface is equivalent to hydraulic gradient in Darcy equation for steady flow of fluidal continuum. Supposing or sampling data of hydraulic coefficient (well data, etc.), therefore, flow rate can be calculated based on Darcy equation. Inclined oil (or gas)/water contacts have been discovered in many fields around the world to date. A hydrodynamic theory has been proposed to explain the physical mechanism of inclined contacts (Hubbert, 1953). Peciko gas field in the Mahakam Delta Province, East Kalimantan, Indonesia gives a typical example of inclined contact. The hydraulic gradient governing the Peciko gas field is caused by the hydraulic gradient between its adjacent hydrostatic domain and a highly over-pressured domain. The present average flow rate in the gas zone is estimated to be 3.1 mm/year. This value is less than 2.5 cm/year of subsurface fluid migration in the North Sea estimated by England et al. (1987). The size of a water molecule (2.1 × 10-10 m) indicates that the slow flow means almost no flow on average. Considering that gas accumulation in the Peciko gas field plugs the paths of fluid migration over the field, the present fluid flow averaged over the field should be less than the normal water saturated environment. Under some assumptions, the flow rate would be 3.1 mm/year; this estimate is consistent when averaging those of sandstone and mudstone.
Silica mineral phase transformations in addition to mechanical compaction and cementation, affect burial diagenetic processes in diatomaceous sediments. Thus, the porosity and/or permeability of siliceous sediments abruptly change with the transformations. Among these diagenetic changes, the transformation from opal-CT to quartz is the most important process in petroleum geology. In this transformation process, permeability improves due to the improvement of pore connectivity induced by the net-like development of new pores. If enough silica is supplied during the process, the silica attaches selectively to portions with high permeability. In this process, the formation of chert just below the transformation boundary is possible. Chert is the most brittle of siliceous rocks and is capable of forming fractured reservoirs. If insufficient silica is supplied, quartz porcelanite with a low clay content shows high reservoir quality within the one-thousand meter interval below the transformation depth. If a clay-rich layer is interbedded in that interval, mechanical compaction progresses selectively in the layer. As a result, opal-CT porcelanite directly above the transformation boundary and clay-rich quartz porcelanite layer below the boundary seal hydrocarbons that have migrated from deeper zones. Meanwhile, if the basin uplifts and the transformation depth becomes shallower than about 500 meters, the permeability of opal-CT porcelanite above the transformation boundary could be as high as the permeability of the quartz porcelanite below it. In that case, opal-CT porcelanite directly above the transformation boundary does not provide an effective seal.
The sonic log provides a good compaction curve that shows the porosity of sediments reducing with depth. Sonic-determined compaction curves from 150 wells in the Niigata Sedimentary Basin are classified into three types: Nagaoka-heiya, Niigata-heiya, and Takada-heiya. The Nagaoka-heiya type is the most common compaction curve in Niigata with Δt (formation interval transit time) decreasing continuously with depth. The Niigata-heiya type shows less compaction with Δt decreasing more gradually. The Takada-heiya type shows more compaction than the Nagaoka-heiya type at depths greater than 1500 m. Compaction is controlled mainly by burial depth, but also by time and temperature. No clear relationships are observed between compaction rate and overpressure or sand/shale ratio. The Niigata-heiya-type wells, which show lower compaction, are distributed in areas with young and rapid sedimentation of more than 2000 m/m.y. Rapid sedimentation may cause insufficient dewatering from pore spaces, in turn causing overpressure and undercompaction. However, the Niigata-heiya-type wells do not show distinctive overpressure, so insufficient dewatering does not explain low compaction. Mechanical rearrangement of sedimentary grain framework takes a relatively long time, and 1 m.y. is not long enough for complete rearrangement of 2000 m thick sediments. Higher temperature accelerates chemical processes such as diagenesis and cementation. Wells of the Takada-heiya type are believed to have been exposed to higher thermal gradients between 16 Ma to 9 Ma, causing rapid compaction and higher maturity of source rocks. Several normal compaction trends have been established for each area in the Niigata Basin. The amount of eroded strata at each well was estimated by referring to the normal compaction trend, and an erosion map of the Niigata basin was made. The hypocenter of the 2004 Mid-Niigata Prefecture Earthquake was located 13 km below the Araya Anticline. Estimated erosion is concordant with the shape of the anticline and deformation of the earthquake. Its cross-section and geohistory diagram indicate the Araya Anticline was initiated at about 1.3 Ma by an inverse movement of the Inokurayama Fault, where a normal fault was reactivated as a reverse fault, and 1600 m of sediments were subsequently eroded.
Groundwater dating was applied to characterize the groundwater flow in the West coastal area of the Miura Peninsular belonging to Neocene sedimentary rock. At the location of the borehole, the Miura Group is distributed from the surface to a depth of 210 m, and the Hayama Group is distributed at depths greater than 210 m. The hydraulic conductivities of the Miura Group and the Hayama Group are 1 × 10-7 m/s and 1 × 10-9 m/s, respectively. There are significant differences between the Miura Group and the Hayama Group. 14C concentration is high (40 to 60 pmC) in the Miura Group and is low (10 pmC) at upper part of the Hayama Group. 36Cl/Cl is equivalent to modern sea water in the Miura Group and in-situ equilibrium in the Hayama Group. 4He concentration is equivalent to atmospheric equilibrium at the upper part of the Miura Group. 4He concentration increases with depth and is constant at 2 × 10-5ccSTP/gw in the Hayama Group. 4He concentration of 2 × 10-5ccSTP/gw is equivalent to 7 Ma accumulation of in-situ production. δ37Cl is equivalent to SMOC in the Miura Group and decrease with depth in the Hayama Group. This implies that advection is dominant in the Miura Group and diffusion is dominant in the Hayama Group. These results are consistent with each other and indicate the difference in groundwater mobility between the Miura Group and the Hayama Group. The difference in groundwater mobility is also supported by: (1) the difference in hydraulic conductivities of the Miura Group and the Hayama Group, (2) fresh water is dominant in the Miura Group and seawater is dominant in the Hayama Group, and (3) the shift of δ18O due to water-rock interaction in the Hayama Group. It is confirmed that groundwater dating provides useful information to evaluate the groundwater flow in a coastal area. This multiple approach could be useful to obtain reliable results.
Numerical simulations have been the most effective method for estimating flow pattern, flux, and flow velocity of the groundwater to precisely characterize large-scale groundwater systems. Spatial modeling of the 3D distribution of hydraulic conductivity over a study area is indispensable to obtain accurate simulation results. However, such spatial modeling is difficult in most cases due to the limitations of hydraulic conductivity data in terms of their volume and location. To overcome these problems and establish an advanced technique, we adopt geostatistics and combine a fracture distribution model with measured conductivity data, selecting the Tono area situated in Gifu Prefecture, central Japan for the field study. The size of the main target area covers 12 km (E-W) by 8 km (N-S), with a depth range of 1.5 km, and it is chiefly underlain by Cretaceous granite. Because the distribution of 395 hydraulic test data acquired along the 25 deep boreholes was biased, the data values were compared to the dimensions of simulated fractures using GEOFRAC. As a result, a positive correlation was identified. Using a regression equation for the correlation, hydraulic conductivity values were assigned to every simulated fracture. Then, a sequential Gaussian simulation (SGS) was applied to construct a 3D spatial model of hydraulic conductivity using those assumed values and actual test data. The plausibility of the resulting model was confirmed through the continuity of high and low permeable zones. The next step is a groundwater flow simulation using MODFLOW and the model. The simulation results were regarded to be appropriate because distribution of hydraulic head, locations of major discharge points, and anisotropy of hydraulic behavior of the Tsukiyoshi fault correspond to the results of observations. The most noteworthy feature detected in the groundwater flow model from the simulation results is that descending flow, horizontal southward flow at depth, and ascending flow are formed from recharge to discharge areas passing through the Tsukiyoshi fault, which agree with the configuration of the continuous simulated fractures.
Research on coseismic and postseismic groundwater pressure changes is important for evaluating long-term groundwater stability in seismically active regions such as Japan. It is also important for preventing and reducing earthquake-induced geological disasters such as landslides. This study reviews literature and clarifies abrupt coseismic changes and subsequent postseismic gradual changes in groundwater pressure. Coseismic changes result from ground shaking and coseismic crustal deformation. Postseismic changes are gradual changes, which can be described by a diffusion equation with new initial values and boundary conditions due to ground shaking and coseismic crustal deformation. The effects of crustal deformation on groundwater pressure appear to be limited in the vicinity of the seismic source region, although that of ground shaking can remain in distant areas. Because a variety of factors and conditions affect earthquake-related groundwater pressure changes, each should be investigated to more accurately clarify the effects on coseismic and postseismic groundwater pressure changes. This research may contribute toward clarifying seismicity mechanisms triggered by distant earthquakes.
It is well known that longitudinal wave velocities and attenuation coefficients in fluid-saturated porous rocks vary with frequency. Evidence comes from experimental measurements and the poroelastic theory using Biot's model. A number of experimental studies in the literature report either direct or indirect evidence of elastic wave dispersion in water-saturated porous rocks. Biot's model predicts a low-frequency sound speed, followed by an increase to a higher sound speed beyond a transition frequency. Biot's model of large-scale mechanisms explains well the dispersion observed in high-porosity rocks, but the predictions of Biot's theory for dispersion are usually lower than measured values by some orders of magnitude. Measurements show that the Biot's model alone does not adequately explain observed velocity dispersion. As a result, much research has been devoted to modifying some aspects of the theory to include additional dispersion mechanisms. Viscoelasticity due to the creation of a local fluid flow approach was devised to represent an additional dispersion mechanism to Biot's model, because Biot's theory ignores the effects of fluid distribution heterogeneity within rocks on their seismic properties. This approach focuses on losses that occur due to the local flow of pore fluid in an individual pore when it undergoes deformation caused by passing longitudinal waves. Several applications of local flow effects can be found in the literature and the squirt-flow is generally known as a dispersion mechanism resulting from pore pressure relaxation caused by viscous flow. However, it is difficult to construct a quantitative model of local flow phenomena because it has to address rock on a microscopic pore-scale level. So far, there has been no consensus on the role of a mechanism that adequately predicts the observed dispersion properties of real rocks. The main purpose of this paper is to provide further insights into the nature of squirt-flow phenomena. Sedimentary rock and crystalline rock specimens were tested using longitudinal waves for differences in velocity dispersion phenomena observed in each specimen, and we examined whether we can quantitatively explain the observed magnitude of dispersion in a variety of rocks. The measurements and theoretical argument show that the squirt-flow model can be applied to observed experimental data. Moreover, it is clear that we can estimate the permeability of rock specimens and artificial porous media using seismic wave dispersion characteristics based on the squirt-flow model.
One of the key issues in the hydrogeologic characterization of sedimentary formations is uncertainty about fluid pressure anomalies that might be generated by chemical osmosis. Groundwater flow under the influence of chemical osmosis is dominated by salinity supply conditions at the boundaries of a semi-permeable formation, as well as by the properties of formation media related to solute and fluid migrations. To obtain chemico-osmotic, diffusion, and hydraulic parameters from a single rock sample, this study devised a laboratory experimental method consisting of permeability and chemical-osmosis experiments, and developed an experimental system capable of performing both experiments on a natural rock sample in sequence under in situ effective stress conditions. The experimental method devised was successively applied to determine the parameters of siliceous mudstone taken from the Wakkanai formation in the Horonobe research area of the Japan Atomic Energy Agency. In the chemical-osmosis experiment, a pressure of almost 100 kPa (about 10 mH2O) was induced by chemical osmosis under a NaCl concentration difference of almost 0.45 mol/L. A series of numerical examinations was performed using 1D hydrogeological models, which revealed that the evolution of osmotically-induced fluid pressure, the duration of chemical osmosis, and the direction of net water flux in semi-permeable formation might depend on salinity supply conditions at the boundaries of a semi-permeable formation. A set of type curves for salinity and pressure profiles in the semi-permeable formation were also obtained to facilitate a preliminary examination of the possibility of chemical osmosis in sedimentary formations. The analytical development was performed using a dimensionless number representing the ratio between time constants of pressure propagation and solute diffusion. Therefore, the results are useful for screening formations where chemical osmosis might exist.
Dr. Kinji Magara was born on 23 April 1936 in Fukui Prefecture, Japan, and passed away on 20 February 2007 in Victoria, British Columbia, Canada. Dr. Magara invented “cross-plot of Δtsh of mudstone (P-wave transit time in micro-second a foot) versus burial depth” to analyze mudstone compaction in its burial history, and extensively studied the burial profiles of Δtsh of the Niigata Sedimentary Basin. He established methods to analyze overpressures in mudstones and the primary migration of oil and gas in burial history. His “downward migration theory” in primary, oil and gas migration was widely accepted over the world. He successfully applied his method to various basins over the world. His analyses should be called “basin analysis” in today's usage. His activities can be divided into four periods. [The first period] 1959∼: graduated from Faculty of Science, Kyoto University (majoring in geology); immediately joined Teikoku Oil Company; later moved to Japan Petroleum Exploration Company, Ltd. (JAPEX) During his works in the companies, he worked as a well-site geologist in the Niigata basin. He was awarded doctor of science from Kyoto University. [The second period] 1967: invited by Canadian National Geology Research as a research geologist, and was on leave from JAPEX for one year; later moved to Calgary, Alberta, Canada, and left JAPEX. 1969: joined Imperial Oil Company (Exxon Canada) as an exploration specialist. 1977: accepted the position of Professor of Reservoir Studies Institute, Texas Tech University. 1979: moved to Bureau of Economic Geology, the University of Texas at Austin to be a research scientist/adjunct professor. [The third period] 1981: moved to King Abdul-Aziz University of Saudi Arabia as a professor. 1992: moved to United Arab Emirates University, and became Professor of Natural Sciences. [The fourth period] 1995: moved to Tsukuba University to be a professor of the course of international development engineering. 2000: accepted a position of professor of Northeast Asia Studies, University of Shimane. 2007: passed away on 20 February, and awarded the professor emeritus from University of Shimane on the date. Dr. Magara was successful in analyzing various, petroleum geological phenomena in the first through third periods. He published three books to sum up his results of studies. His works in the first through third periods are still referred in many works. He changed his targets of studies in the fourth period to international development engineering and comparative analyses of relationships between scientists or engineers and specialists of humanities.