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A CASE STUDY ON THE INTERPRETATION OF GRAVIMETRIC AND MAGNETIC DATA FOR DECIPHERING GEOLOGICAL STRUCTURES WITH OIL AND GAS

Laurențiu Asimopolos

Abstract

In this paper we present a case study on the identification and contouring of a gas deposit, placed mainly in Pontian formations. This study is based on some geological landmarks as well as the interpretation of filtered gravimetric and magnetometric data. The gas deposits determine minimum gravimetric anomalies, under the specific geological conditions of the Transylvanian Basin. Such gravimetric minimum is determined on the one hand by the rise of the salt, and on the other hand by the mass deficit within the salt dome in relation to the reservoir water, from the adjacent synclinal areas. The second element that completes in solving the interpretative problem is the presence of andesitic epiclastites with age Kersonian - Meotian. Corroborating these two aspects, it turns out that the presence of structural elements, containing gas accumulations determines gravimetric minimum anomalies, in the front of the Eastern Carpathians from their external curvature (Focsani Depression), with the main source in the Pontian deposits, conjugated with magnetic anomalies due to the structural elevation of the Kersonian - Meotian epiclastic formation. The presence of gravimetric minimum anomalies are all the more significant because at the Miocene - Pliocene limit there is a density contrast of approximately + 0.1 g / cm?. This density contrast is determined by the higher degree of cementation of the Pontian formations, compared to the Dacian formations. The problem of possibility detecting the water/gas limit, was based on a weak horizontal gradient of maximum gravity around the central minimum. The gravels of Candesti, which mainly contain components of Kliwa sandstone with unconsolidated sandy matrix, have a density of 2.33 g / cm?. The thickness of the Candesti gravels being variable masks the gravimetric minimum due to the presence of the gas deposit. The upper part of the Candesti gravels formation is easily mapable on topographic maps, due to the fact that the respective formation is reflected by specific geomorphological characteristics, as well as by the absence of any vegetation. In contrast, the lower part of the Candesti gravel formation, can only be detected seismically. With these data we have made the gravimetric correction of the effect of the gravel of Candesti.

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