THE author first states the general conditions under which gas may occur in oilfields and then proceeds to give the results of his studies of the occurrence of gas in the Comodora Rivadavia oilfield in the Southern Argentine. The geology of the Comodora Rivadavia field is briefly sketched and the oil-bearing horizons noted and described.

Certain anomalies noted with regard to the distribution of gas in the cover rocks of the field, the Pehuenche formation, are referred to. Accumulations of gas, worthy of note, occur irregularly in these beds at few places. The author does not accept the view that this is due to lithological conditions being favourable in such places while being unfavourable over the-greater part of the area covered by these deposits. He states that the Pehuenche beds are such that the gas cannot have originated in them and that it must have reached that formation from lower horizons.

Gas may be freed from oil-impregnated strata by changes in thermodynamic conditions, temperature and pressure. The effects of rise of temperature as a function of depth, the neighbourhood of magmatic masses and the presence of faults are discussed: first, with especial reference to the Comodora Rivadavia field, where he notes that in neighbouring wells one may be extremely rich in gas while in the next gas may be present in negligible quantity; secondly, in more general terms. Finally, he stresses the importance of studying the dislocations present in the cover beds of an oilfield area and connects the presence of these anomalous accumulations of gas with such dislocations.

Dans le cas général une Coupe complète d'un complèxe pétrolifère peut renfermer les séries suivantes des sédiments:

  1. Le recouvrement improductif.

  2. Les dépôts renfermant, de préférante dans sa partie supérieure, les accumulations du gaz, dérivé des pétroles liquides sous-jacents. Ce gaz est secondaire, car il se trouve dans les roches-magasins qui n'appartiennent pas à la même période de sédimentation, qui a donnée lieu à des roches-mères.

    C'est à l'état d'occlusion dans les espaces interstitiels de la roche que se trouve ce gaz.

  3. Les sédiments naphtogéniques proprements dits, qui peuvent bien entendu, renfermer non seulement les accumulations du pétrole, mais aussi celles de gaz, qui est dans ce cas le gaz primaire, car il se trouve dans le lieu d'entassement originel des débris organiques.

    L'état dé gisement de gaz dans ces dépôts de sédimentation primaire, est beaucoup plus compliqué. Ce gaz peu se

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