One of classic but still a controversial problem is the determination of reservoir descriptions by pressure decline or production history curves. The data available from gas or oil reservoirs during production are oil, gas and water production histories, pressure history and history of produced hydrocarbon compositions. These histories are affected by permeability, porosity, reservoir size, original gas and oil in place and initial pressure, so that it may be possible to determine these reservoir descriptions from the profile of the various histories. Some previous authors ran to such an extreme as to determine heterogeneous permeabilities of a reservoir dividing it into more than 6 or 7 blocks. Another author went too far in saying that they can determine rock compressibility and water influx constant in addition to gas in place and aquifer volume only from gas production and pressure histories. But is it always possible to determine so many reliable parameters from such little information though most of the field data and mathematical models include various errors? This paper answers a certain aspect of this question applying a systematical error analysis to a parameter determination problem for a gas reservoir whose production histories are known.

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The simplicity and versatility of the Kalman sequential filter has establish it as a fundamental tool in the analysis and solution of linear estimation problems. Petroleum engineers recently recognized it as a potential tool for regression analysis and applied it to non-linear problems. However numerical experience has shown that the Kalman filter algorithm is sensitive to computer round-off and that numeric accuracy sometimes degrades to the point where the results cease to be meaningful. In order to avoid this trouble, Potter proposed to decompose the covariance matrix into a square root covariance matrix.
In this paper, the Potter's linear algorithm is applied to two non-linear problems, one of which is non-recurrence type and the other is recurrence type. The linearization of the non-linear functions proposed in this paper appears to be proper to the respective type mathematical models. In addition, two other methods for parameter determination are tested in this paper, which are suitable to mathematical problems with discontinuous functions.

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The author has performed isothermal pyrolyses of kerogen at low temperature (_??_300°C) which are accompanied with the generation of bitumen. Kerogen samples were isolated by acid treatment from the mudstones which are accepted as the most excellent source rocks of the Tertiary in Japan. The thermal products were measured by the method of conventional organic analysis of source rocks.
The author made reaction models for the generation of the products. Kinetic parameters in the models were calculated by the use of multiple regression analysis of the pyrolysis data.
The results are as follows: (1) Thermal decomposition of kerogen consists of two types of process. One is that which takes part in the formation of petroleum-like components, and the other is that which does not have relation to the generation of petroleum. (2) Generations of petroleum-like components are not simple 1st order reaction, but are well simulated with the models which consist of several 1st order reactions, parallel and independent. (3) Concerning the 1st order reaction plots, that of heavy hydrocarbons (HC) shows more linear pattern in comparison with those of total organic extract (TOE) and O-N-S compounds+HC (ONS'). These factsshow that the process of the generation of HC is simpler than those of TOE and ONS'. (4) The energy levels which correspond to modes of the distributions of product forming potentials for HC, ONS'and TOE become higher in the order named. (5) It seems that quantities of gaseous and liquid products have relation to the petroleum forming potential. (6) The ratio of the quantity of sample [represented by the quantity of organic carbon (Corg)] to the volume of reaction container (VRc) have an affect to some extent upon the generation of thermal decomposed products both in qualitative and in quantive terms. (To be continued)

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In this report, the author attempts to restore, as a function of geologic time, the generation of petroleum-like components from kerogen in mudstone sample. The restoration is performed with the following assumptions: (1) Kinetic parameters calculated from pyrolysis data of kerogen could be ap-
plied to the natural processes of the generation of decomposed products. (2) Geothermal gradient and surface temperature have not changed through geologic time.
The author also applies the same kind of simulation to neighboring areas where the same formations should be expected and should have different thermal histories. In this procedure, the following assumption is added: (3) There is no lateral change in the characteristics of organic matter and lithofacies at
deposition all over the evaluating areas.
When there seems to be little migration of heavy petroleum-like components, the results of simulation could be compared with the analytical data of mudstones at each evaluating point.

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The petroleum geology in China has been reported by several authors and the outline of the geologic conditions of the oil and gas fields in China can be understood through the published papers. It was reported that more than 50% of the Chinese petroleum production was from the Ta-ch'ing oilfield
in the Sung-liao basin, Northeast China, in which the non-marine sediments of the Mesozic and Tertiary ages were deposited. Thus, the non-marine sedimentary basins rated highest in petroleum production in China. However, it is considered by the present author that the marine and paralic sediments of the Paleozoic and Mesozoic ages are of importance to the future petroleum exploration in China, especially in the inland regions of the continent. From this standpoint, the geological and geochemical aspects on the following items are described.
1) 382 oilfield in Tibet.
This oilfield is located on an elevation of about 4, 000 meters and some 300km north of Lasa. Petroleum has probably been produced from the marine sediments of the Paleozoic and Mesozoic ages or the Tertiary lacustrine sediments.
2) Newly discovered oil and gas reservoirs in the Southwest Tarim basin.
In the new oilfield located in between Aksu and Kashgar, Sowthwest Tarim basin, petroleum is probably produced from the marine and paralic sediments of the Mesozoic and Tertiary ages.
3) Geologic horizons of oil and gas reservoirs in the Central-Eastern China.
The commercial petroleum production in the Jen-ch'iu and Sheng-li South oilfields was reported from the marine Paleozoic reservoirs. In the Szechwan basin, hydrocarbons have been produced from the marine Permian and Triassic reservoirs and from the non-marine Jurassic ones. In the Ch'ien-chang oilfield
in the Tung-t'ing basin, the petroleum production is from the Devonian and Carboniferous reserviors. Thus, in the sedimentary basins along Yang-tze River, the marine Paleozoic and Mesozoic sediments including carbonate rocks, sandstones and shales probably are of importance to hydrocarcon exploration, bec-
ause, it was reported by the Chinese exploration geochemists that in China even the Sinian carbonate rocks still keep up the potential for petroleum generation.
4) Natural gas in the coastal areas along the East China Sea.
Many gas producing locations were found in the area. In the sedimentary basin near Shang-hai, the thickness of sediments ranges from 4 to 6km or more. It is considered by the author that the marine Paleozoic and Mesozoic sediments, the probable continental and paralic Tertiary sediments and the paralic
Quaternary sediments are the main targets for hydrocarbon exploration in the Shang-hai area. In addition to this, in the South-East China, small but many sedimentary basins filled with the Mesozoic lacustrine deposits of only from several ten to several hundred meters thick are the economic exploration targets.
As a conclusion, it is supposed that the marine and paralic sediments of the Paleozoic and Mesozoic ages in China have higher potential in hydrocarbon resources than have been estimated.

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[in Japanese]

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