“Shale Rocks” have been investigated in detail since 1970's as Source Rocks and Seal Rocks for conventional oil and gas, which promoted the understanding of “Shale Rocks”. In recent years, unconventional oil and gas, especially Shale Oil and Gas have been paid much attention as new resources. This situation is driven by the progress in enhancing technology such as hydraulic fracturing and higher oil and gas prices. Shale Oil and Gas are actually to produce remaining oil and gas in matured Source Rocks, which could not be expelled during geological history. Therefore, the knowledge derived from Source Rocks for conventional oil and gas should be very useful for the Shale Oil and Gas development and production. Not all Source Rocks are good for the Shale Oil and Gas. In order to be economical in current situations, the production rate after fracturing is the key. High production rate should be accomplished from the “Shale Rocks”, which have much concentration of oil and gas and brittle character rich in brittle minerals such as quartz and carbonate. Therefore, the “Shale Rocks” good for Shale Oil and Gas are older (Paleozoic and Mesozoic) marine Source Rocks with high TOC and maturity. In order to save costs, the well for Shale Oil and Gas should be shallower. However, in order to generate oil and gas in Source Rocks, they have to bury more than 3000m, which is too deep for the Shale Oil and Gas businesses. Therefore, the Source Rocks should be uplifted shallower after the generation, which is likely to occur in onshore areas. The conditions discussed above limit the Shale Oil and Gas businesses within North American Continent now. However, they can expand to other areas in the future by using current knowledge of Source Rocks.
Stress analysis based on borehole failures is becoming a common procedure to evaluate not merely in-situ stresses but fault seal and fluid flow, to maximize the success rate of exploration and production capability. The mechanics of borehole failures, breakouts and drilling induced tensile fractures (DITFs), can be explained by stress heterogeneity generated around the wellbore as an open space in the stressed rock mass. The natural stress field is constrained by the Coulomb frictional sliding, and the rocks break when the stress condition fulfills an appropriate failure criterion. Thus the characteristics of the borehole failures and rock strength can be used to calculate the in-situ stresses. The stresses sometimes fluctuate around discontinuity surfaces, such as faults and fractures, suggesting that these planes may be tectonically active and can be conduits of fluids.
Change of residual hydrocarbon gases in mudstones and pelitic metamorphic rocks was investigated to evaluate the retention of methane in shale rocks during diagenesis and metamorphism. The samples were collected from borehole MITI-Mishima and outcrops in Shimanto belt, Chichibu belt, and Sanbagawa metamorphic belt exposed in Shikoku region. Concentration of residual methane normalized to total organic carbon content begins to increase when mudstone porosity reaches about 5%. The increased methane concentration at mudstone porosity less than 5% suggests a significant retention of methane in mudstone matrix. The high concentration zone of residual methane corresponds to maturity levels of vitrinite reflectance (Ro) =2 to 3%. Residual methane concentration significantly decreases in the higher maturity levels (Ro>4%), suggesting the graphitization of methane. The maturity levels of Ro=2 to 3% with the highest residual methane concentration is expected to be the favorable stage for shale gas formation, although host rock lithology such as the storage capacity and fissile nature are also related to enhanced shale gas recovery. Many parts of accretionary prism in Shimanto belt are at these maturity levels of Ro=2 to 3% favorable for shale gas formation. Accretionary prism formed under the compressional stress suggests development of gas seal structures and closed fracture systems convenient for shale gas retention, although shale rocks in Shimanto accretionary prism are comparatively poor in organic matter. Several thermogenic gas seepages in Shimanto belt would be another indication of shale gas formation in accretionary prism distributed in wide area along Pacific side of southwest Japan.
The Onnagawa Shale is bio-siliceous, Monterey-like and the main source rock in Japan, particularly in Akita basin, with fairly high potential. It is 500m thick in average and has TOC of 2% in average, up to 5% in part, and HI of 500 mg/gC typically. The Onnagawa is under review in the light of Shale oil/gas, comparing the Monterey shale. As the first step, we have reviewed the Yuihara-Ayukawa field, where the largest commercial oil & gas production from the Onnagawa Shale is ongoing. In the field, various reservoirs, including Dolerite, Tuff and quartzose porcelanite probably fractured, seem to have a common gas-oil contact. In other words, they constitute one pressure system. The maturity of the hydrocarbon in this system is much higher than that of source rocks close to the reservoirs, which indicates normal migration process. We also have sub-commercial production or intense oil show from relatively tight reservoirs that is typically alternation of porcelanite and tuff. The maturity of the oil, however, is lower than that of commercial oils and close to that of early-matured source rocks around. In summary, we have two different hydrocarbon accumulations in one filed. One is productive, mature and probably migrated, and another is sub-commercial or non-productive so far, less matured and indigenous. In addition, we have other similar examples in the Akita basin and we think this less-matured oil group as a part of basin-wide shale oil system, where our field is located in the transition zone just above oil generation window. We expect a potential with an order of 100 million bbl in this basin including the kitchen. Now the first pilot test is being planned to apply horizontal drilling and massive stimulation treatment to “shale oil or tight oil” in this basin.
The tephra which spreads and deposits in wide areas by volcanic eruption is one of the most important time makers for the paleoenvironmental researches in and around the Japanese Islands. However, the source and eruption age of many tephras is still unknown (Machida 2011). The study area, off Joetsu of the Japan Sea, has great advantage to establish tephrostratigraphy because the various tephras from the Japanese Islands and Ulreung-do Island are probably distributed and because sedimentation rate in the study area is high since the area is situated close to the continental shelf and Japan Alps. This report revealed the tephrastratigraphy off Joetsu based on the identification and correlation of the tephras layers collected from the long core samples obtained by MD 179 survey. The tephra samples were collected from the six piston cores and two gravity cores collected at geomorphologically different places off Joetsu, and at the submarine valley off Sado Island. Each sample was listed in the mineral composition and the morphologic feature of volcanic glass shards, and the chemical composition of 20 shards of volcanic glass measured by microprobe analyzer (SEM-EDS). The total of 95 tephra samples were classified into 32 kinds of tephras based on the shape and the chemical composition of volcanic glass along with heavy-mineral combination. Then 12 kinds of them were identified and correlated with the well-established tephras, although the source and age of the other kinds of tephra were uncertain.
It is well known that horizontal drilling and hydraulic fracturing are key technologies to carry the shale gas revolution forward to success. Efforts in enlarging reservoir contact are needed to realize commercial production from shale reservoir whose permeability is supposed to be in the order of nano-Dary to micro-Darcy. To design mud system, to keep wellbore stability and/or to optimize hydraulic fracturing fluid and pumping schedule, knowledge about rock mechanics and stress environment is quite important. A technical area of analyzing, integrating and interpreting such knowledge is called “geomechanics”. In this paper, we describe geomechanics from various aspects by focusing application to hydraulic fracturing technology. We review mechanical property of shale and some important parameters such as Young's modulus, Poisson's ratio, UCS, hardness and brittleness. Then, we show how those parameters impact hydraulic fracturing. A geomecanical models which consist of mechanical faces, mechanical properties and stress information is briefly explained. A case study of microseismic monitoring in Barnet shale field is introduced to reveal created hydraulic fracture patterns and those relationship to production that are significantly affected by pre-existing natural fracture system.
Shale gas production in USA has grown rapidly in recent years. The detail production mechanism from shale rock is still not clearly investigated since it has a relatively short production history. This paper shows summary of shale gas exploration and development, project evaluation method and geological approach to understand shale evaluation based on public information. Various geological feature of unconventional play should be considered for the geological evaluation. Furthermore unexpected problems for shale gas development are shown, which came up with the growth of development and production activities in USA.
The On'nagawa Formation in Akita Sedimentary Basin is composed of siliceous mudstone and volcaniclastics. The volocaniclastic facies have been productive reservoirs of oil and gas fields in the Neogene Akita Basin, North Japan. The mudstone facies have been recognized to be potential source rocks for hydrocarbons pools in the reservoirs. After a long period of oil and gas production in the basin, a potential exploration concept of a new play in the siliceous rocks is required to keep the productivity of the basin. This study reports a basic description of physical properties of the siliceous mudstone facies in order to provide a basis for the idea of shale play in the On'nagawa Formation. Wireline logs (GR, Δt, ρb) are basic data for our description of the physical rock properties as well as descriptions of cutting lithology.
Eleven molluscan species were recovered from 11 cores collected from off Joetsu during MD179 cruise of R/V Marion Defresne. They include three thyasirid species Conchocele bisecta, Thyasira tokunagai and T. sp. which are characteristic species of chemosynthetic fauna. The occurrence of these species corresponds to the horizon below and above the LGM (Last Glacial Muximum), suggesting that the seepage of fluid including methane not at the coldest time. From the bed corresponding to the LGM, many mollusks including two deposit feeders as Robaia robai and Yoldiella philippiana are concentrated probably because of rich organic matter during the LGM. This is also another criterion which the endemic species of the Japan Sea, Robaia robai could survive the deteriorated environment in deep-sea during the ice age.
Several long piston and gravity cores of the Japan Sea sediment are recovered during the cruise of the R/V Marione Dufresne in 2010 (MD179). Fundamental core description is presented here to share common information of the collected sediments that will be served for various analytical studies. In addition to the description on-board, detailed observation is performed by the images of the multi-scanner and the X-ray CT scanner. The recovered sediments are mainly consists of alternated dark- and light-colored silty mud with intercalations of tephras and sand laminations in some cores. Dark layers are commonly associated with fine parallel laminations that is traditionally called thinly laminated (TL) layer. The TL layers have been used both in estimating the change in paleooxygenation of the bottom of the Japan Sea and in making correlation with other cores. The observed TL layers in the cores are numbered following some of the foregoing papers depending on the identified tephras. The identified TL layers are described briefly.
Several long piston and gravity cores of the Japan Sea sediment are recovered during the cruise of the R/V Marione Dufresne in 2010 (MD179). Fundamental core description on-board and the images of multi-scanner and X-ray CT scanner is presented here to share common information of the collected sediments that will be served for various analytical studies. The sediments of the most cores are disturbed more or less by faults or folds even if they were very minute and the stratigraphic succession is kept in order. The TL layers are hardly identified in some horizons of several cores whose sediments are disturbed by slumping and/or brecciated by debris flow. MD179-3312 is selected to compare with a published figure of columnar section from the Japan Sea, the secular changes of the δ18O of both seawater and the ice sheet of the northern Greenland.