Thick piles of basalt of middle Miocene Aosawa Formation are spread extensively in the underground of the Yurihara Oil and Gas Field, back-arc side of the NE Japan arc. Another nearly correlative basalt (Taikura Basalt Member of Hatamura Formation) is exposed locally on a hilly land to the east of the oil and gas field. These two basalts show some significant difference in thickness, lithofacies, environment of eruption and petrological characteristics.
The basalt of the Aosawa Formation erupted on the subsiding submarine basin and consists of relatively undifferentiated (total FeO/MgO=0.9-1.4) tholeiitic basalt. It is similar in mode of occurrence and petrological characteristics to other correlative basalts within the Akita-Yamagata oil field basin in the back-arc region of the NE Japan arc. On the other side, the Taikura Basalt Member was accumulated on land or in shallow water, and is slightly differentiated and rich in TiO₂, K₂O and light REE compared with the basalt of the Aosawa Formation. Both the basalts have some affinities in mode of occurrence and petrological features for the tholeiitic and alkalic basalt in a rift zone such as the Rio Grande rift. Both the basalts were probably formed in a back-arc rift zone during middle Miocene age.

View full abstract

Pore size distribution is one of the most important reservoir rock properties. The purpose of this report is to calculate and evaluate pore size distribution from capillary pressure curves using the centrifuge method.
The core samples used in this experimental work were Berea Sandstone, Abu-Dhabi Dolomite and Japanese Sandstone. Reservoir rocks were frozen using liquid nitrogen and small core plugs were cut and recovered from them.
Capillary pressure of small core plugs was measured using the centrifuge method. Pore size distribution was calculated and evaluated from their capillaly pressure curves.
After analyzing the graphical representation of these data, the following points are reached:
(1) In Berea Sandstone, the slopes of capillary pressure curves were very steep and most of pore radii distributed 4-5μm.
(2) In Abu-Dhabi Dolomite, the slopes of capillary pressure curves were relatively steep and pore radii distributed narrowly.
In addition, irreducible water saturations were low. This is due to the nonwetting characteristic of Dolomite with respect to water.
(3) In Japanese Sandstone, the slopes of capillary pressure curves were relatively gentle and pore radii distributed widely.
In addition, irreducible water saturations were high. This is due to the wetting characteristic of Sandstone with respect to water.
The centrifuge method for the determination of capillary pressure and pore size distribution is proved to be very effective. This is mainly due to the repeatability in conducting capillary pressure experiments and the number of cores that can be tested at the same time using this method.

View full abstract

Succession of the Middle Miocene to Pliocene series distributed in this area corresponds to the standard succession of the Niigata sedimentary basin, but is complex because these series include larger amounts of volcanic rocks than that of type area and still remain unknown.
From detail field and biostratigraphical study, and fission track dating, the succession and age of the Nanatani, Teradomari and Shiiya formations of the Miocene and the Nishiyama Formation of the Pliocene were clarified.
Biotite-hornblende dacite and hornblende andesite (9.0Ma) of the Teradomari Formation are interbedded with siltstones deposited in sedimentary environment of upper to middle bathyal zone and pyroxene andesite of the Shiiya Formation are interbedded with siltstones and sandstones deposited in the same environment as the Teradomari Formation. The Teradomari Formation is considered to has deposited on the Nanatani Formation which has occured as topographical high, resulting in clinounconformity between the two formations. The Nokogiriyama fault, which is main geological structure and is related to the Nigoro anticline, may has began to occur in the Shiiya stage as shown in defferences of lithofacies between both flanks of the Nigoro anticline.

View full abstract

Simulation study was carried out to investigate the behavior of fireflooding and to obtain basic information for designing and/or screening fireflood projects. The simulator used has been reported elsewhere (JJAPT 52, 5, 1987). Parameters investigated were
(1) initial water saturation, (2) absolute permeability, (3) porosity, and (4) thickness of reservoir (as a parameter representing heat loss to the upper and lower strata).
Results are summerized as follows:
(1) The course of oil production may be very much affected by the initial oil saturation.
(2) Absolute permeability of a reservoir may not be sensitive to the total performance but the injection pressure of air.
(3) Porosity is one of the critical parameter of screening.
(4) A thin reservoir may lose heat to the surrounding strata resulting in cease of the combustion reaction.

View full abstract

In the preceding paper²¹⁾, the author presented a new method of generariog directional pseudofunctions automatically by the simulator itself. The method combines the vertical equilibrium concept with the Buckley-Levrett's Fronal Advance Theory. In this paper, additional details of the method which generates the FAP (Frontal Advance Profile) pseudofunctions are described in conjunction with B-L theory. Results of 1-D and 2-D test runs are presented to show that the water-cut performances and profiles of water front obtained by coares-grid models using FAP pseudofunctions are almost identical with those obtained by fine-grid models using rock saturation functions. General discussion on numerical dispersion and pseudo-generation methods are also made focused on field-scale reservoir simulation using a coarse-grid system.

View full abstract

New type of polycrystalline-diamond-compact (PDC) bits are tested for drilling hard rocks in laboratory conditions. Slender cylindrical inserts are used as cutters, so the bits are called as polycrystalline-diamond-insert (PDI) bits in this study. The inserts are 3-3.5mm in diameter, and 5mm in length in the test bit of 6cm in diameter. The inserts are composed of PDC and tungsten carbide (WC). Four types of inserts are tested: PDC sheathed by WC coaxially, PDC coverd by WC eccentrically, semicircular PDC welded with semicircular WC, and circular PDC.
Drilling rate and bit life are measured of nine test bits with different profiles. The bit profile is designed considering inserts distribution pattern, cuttings removal, stable motion of bit, breakage of a center plug of bottom-hole rock and so on. No. 8 bit drilled about 30m of granite, of which compressive strength ranges from 130MPa to 160MPa, with drilling rate of about 2.2m/h at bit weight of 25-15kN and rotating speed of 99rpm. No. 9bit shows high drilling ability of 4m/h in granite, of which compressive strength is about 130MPa, at bit weight of 12kN and rotating speed of 99rpm. The bit has stable drilling rate and torque during long periods, when an adequate combination of bit weight and rotating speed is applied. Low bit weight is apt to wear off inserts faster than matrix, and high bit weight is apt to wear off matrix. At optimum conditions, inserts and matrix seem to wear off slowly keeping the relative height of inserts from the matrix.

View full abstract