Martin Nava Garcia
Comision General de Energia (SEPAFIN).
ABSTRACT
The Geophysical age of a productive formation of drilled wells can be determined through geophysic logs correlation of drilled wells in the Cretaceous area Tabasco - Chiapas and the marine platform Campeche-Yucatan. And through the comparison of the above mentioned logs with the logs obtained in the drilled wells of the Papaloapan basin in the southern part of Veracruz and in the area Reforms of Tabasco - Chiapas, and if we take into account the data obtained from former jobs with emphasis in geological and geophysical aspects; we postulate the hypothesis of the existence of a sequence of structures (similar to those presently under exploitation in the Cretaceous area Tabasco - Chiapas).
These structures are located from mid-state Veracruz, Tabasco and Chiapas , the border of the continental shelf –Campeche - Yucatan , to the coastal region of Quintana Roo , North of Belice and Guatemala going back to the Reforma area. It could be part of a huge atoll, thus increasing oil –reserves in Mexico.
J.A. Czubek, K. Drozdowicz, E. Krynicka-Drozdowicz, A. Igielski
and U. Woinicka.
Institute of Nuclear Physics
ul. Radzikowskiego 152
31-342 Krakow , Poland
Abstract
The knowledge of the true absorption cross section of thermal neutrons for different rock materials and brines is an important factor in the quantitative interpretation of the neutron lifetime logs. This absorption cross section is directly related to the elemental composition of these materials. Geological materials, however, very often contain some admixtues of highly absorbing isotopes in quantities which are still too low to be detected in the usual elemental analysis (like boron, rare earths, etc.). The only way to know that cross section is to measure it on geological samples. Due to the particular features of rock material (heterogeneities, impossibility to reproduce the sample of exactly the same composition and bulk density and with different dimensions, etc.) this is not an easy task using the experimental methods known so far. A new approach to this problem is presented in the paper.
In consecutive measurements the rock sample (having a fixed and well known shape - in our case it is a sphere and the sample is powdered or liquid) is enveloped in shells of a Plexiglas moderator (the neutron parameters of which are known) of variable thickness and irradiated with the pulsed beam of fast neutrons. The die-away rate of thermal neutrons escaping from the whole system is measured. The absorption cross section of the sample is found as the intersection of the experimental curve (i.e. die-away rate vs. thickness of the moderator) with the theoretical curve. The theoretical curve is calculated for a given moderator under the assumption of a constant value of the neutron flux inside the sample. This method is independent of the transport cross section of the sample.
The method has been checked on artificial materials with a well known elemental composition (liquid or solid) and on the natural brine. For the liquid sample the one standard deviation of the measurement is of the order of 2 or 3 capture units, whereas for the solid sample it is less than 5 capture -units. The volume of the sample needed here is of the order of 1000 ccm.