In 1936 OMURA distinguished two tectonic movements, the Dewa disturbance during the Latest Miocene and the Earliest Pliocene and the Oil-field disturbance of the Latest Pliocene, in the inner oil-fields of Northeast Honshu, Japan. He considered the Dewa disturbance is characterized by an epeirogenic movement and oil-traps were completed by the orogenic Oil-field disturbance. In this oppotunity, I explained my personal consideration on the Dewa disturbance. This disturbance is an orogenic movement in the Katsurane substage (Earliest Pliocene), being associated with the geanticlinal upwarping since the Later Funakawan stage (Latest Miocene). Oil-bearing formations in Akita oil-fields, such as the Onnagawa, the Funakawa and the Katsurane, were strongly folded and faulted, with N-S trend, by this disturbance, then oil-traps were formed in anticlinal structures. Some weaker tectonic activities followed in the Later and the Latest Pliocene, but they were rather remote from oil-accumulation. The sedimentation of the orogenic phase of the Dewa disturbance is represented by the Katsurane formation which is characterized by alternation facies of abnormal rudaceous sediments and mudstone. The remarkable differentiation of the Akita sedimentary basin, in which upheaval belts and sinking zones appeared here and there, were caused by the Dewa disturbance. Some large faults had controlled, with long-ranged activity, thrust upheaval in one side of the fault and reverse sinking in another side, through the Tentokuji and the Sasaoka stages (Later Pliocene). In these minor basins, younger formations, such as the Tentokuji and the Sasaoka, formed gentle structure, but the underlying oil-bearing formations were severely folded and faulted.
Basic considerations of upward and downward migrations of fluid in the subsurface have been discussed in part 1 of this paper. In part 2, the thicknesses of the upward and downward fluid movement zones in several schematic examples are calculated. In order to estimate the amount of the downward compaction current, the thickness of the downward zone is related to the porosity decrease during compaction. These calculations indicate that the main downward compaction current would occur at relatively earlier stages of the rapid sedimentation. On the other hand, the main downward compaction current after sedimentation stops would take place just before the shales reach compaction equilibrium, or at relatively later stages of compaction.
In the previous paper a method analyzing quantitatively the effect of hydraulics on the rate of penetration has been suggested as a result of laboratory experiments. In this paper an attempt is made at the application of that method to analyze field data. Tsuchizaki-Oki oil field and Kumoide gas field are chosen as typical ones. Results obtained are as follows: (1) The concept of Bingham's performance line is approximately applied to these fields. It is obvious in Figs. 1 and 4 that a number of penetration rates comes under the performance line and drilling operations are frequently canducted under conditions of unadepuate hydraulics. (2) The changes of penetration rates under conditions of unadequate hydraulics are estimated with applcation of the equaiton of relative penetration rate which is even a tentative one as deacribed in the previous paper, and they are shown by broken lines in Figs. 1 and 4. At some instance an increase in penetration rate may be possible with increase in bit weight, but at another instance it is necessary for increase in penetration rate to improve on hydraulics rather than to increase in bit weight. (3) Separation points of penetration rate from performance line serve as a parameter of hydraulic action. The penetration rate at separation point is related with the impact force which is modified for the effect of a bit diameter, and the relationships between them are appraximately linear as shown in Fig. 2 and 5, of which a general form is as follows : Rb=a+b(D0/D)2M where Rb=penetration rate at the separation poin, M=impact force at the bit diameter D, D0=reference bit diameter, a and b=constants. A quantitative evaluation of the effect of hydraulics may be possible by the use of this relationship. It may be possible, for instance, to estimate an increase in penetration rate attained by replacing a small pump with a large one. (4) The drillability of a formation may be evaluated with two paraneters, i. e., performance slope and primary intercept. Judging from these two parameters, drillabilities of formations are genarally higher at Kumoide gas field than at Tsuchizaki-Oki oil field. (5) It is observed at Tsuchizaki oil field that the penetration rate increases in so-called abnormal pressure formation against a tendency to a decrease in penetration rate with the depth.