Własności zbiornikowe piaskowcowych utworów kambru środkowego strefy Łeby-Żarnowca w świetle analizy statystycznej

Abstract

W badanym kompleksie wydzielono dwa typy litologiczne granularnych poziomów zbiornikowych: typ I - względnie jednorodne serie piaskowców drobno- i średnioziarnistych; typ II – serie przekładańców ilowcowo-mułowcowo-piaskowcowych, z przewagą tych ostatnich. Wyróżniono także tzw. semiekrany, będące formą pośrednią między skałami zbiornikowymi a warstwami uszczelniającymi. Omówiono pionową i poziomą zmienność porowatości laboratoryjnej i geofizycznej (współczynnik porowatości kp) oraz scharakteryzowano własności filtracyjne poziomów piaskowcowych. Wykorzystując statystyczny program EPISTAT, wykonano analizy: rozkładu pojedynczej zmiennej losowej, korelacji i regresji oraz wariancji jednoczynnikowej zbiorów danych: głębokości, porowatości laboratoryjnej, porowatości geofizycznej, przepuszczalności i gęstości. Przedstawiono geologiczne wnioski wynikające z tych obliczeń. The reservoir properties of the Middle Cambrian sandstone deposits in the Łeba-Żarnowiec area in the light of the statistic analysisA very important or sometimes decisive condition to carry out oil exploration works in the Łeba-Żarnowiec area, besides a structural factor, is the knowledge of spatial distribution of the reservoir properties of the Middle Cambrian sandstone series - a main perspective and productive horizon. The Middle Cambrian formation is bipartite. The lower part represents monotonous claystone and siltstone complex with thickness about 150 m. The upper complex formed quartzitic sandstones with a various degree of silification, generally fine- and medium-grained, well rounded, interbedded and laminated with claystones and siltstones. Its thickness ranges from 47.5 to 132.5 m. Two lithological types of granular reservoir horizons have been distinguished in the sandstone complex:- type I - horizons with relatively homogenous interior structure representing well sorted, fine-grained, poorly clayey massive and thick-bedded sandstone,- type II horizons of interbedded claystones, siltstones and sandstones with sandstones domination. Sometime particular beds of sandstone with thickness not exceeding 1-2 m are analogous to the type I sandstone.To complete the characteristic of reservoir properties of the sandstone series, due to poor quantity of core material, the geophysical coefficient (kp) counted from the neutron - gamma logs was used. Sandstones are characterized by low various porosities (Tab. 2, Figs. 1-3). It has been stated that boundary value of porosity that permits to qualify the sandstone series to the reservoir horizons is 5%. Horizons of type I are characterized by values of mean weighted porosity varied from 5.0 to 9.16%; type II from 5.0 to 7.23%. Permeability of sandstones vary in a wide interval from 0 to 24 mD (Tab. 3, Fig. 4). The highest values (10-20 mD) were stated in type I sandstones, especially in horizons with larger thickness. Type II sandstones generally are low permeable < 0,1 mD, occasionally reaching 5 mD. The statistic analysis comprised the following petrophysical parameters of sandstones (regarding the depth of occurrence for each sample): laboratory porosity, geophysical porosity, permeability and density. Calculations were carried out with the IBM/PC-XT microcomputer using EPISTAT - a ready statistic program. Structure and formal limitations of this program do not allow to carry out the cumulative analysis of the total set of the determined parameter, but only its analysis in a single borehole. The analysis of correlation and regression of particular pair of parameters was done using the Pearson's linear correlation coefficient (R), testing its real value on the probability level a = 0,05 (Tab. 4) supplemented by the analysis of single parameter random variable distribution. To statistic comparison of two sets of porosity the unifactor variance analysis was used. It allowed to check the equality of mean value of both sets, therby their similarity (Tab. 6). It was showed that in the case of detailed characteristics of the Cambrian sandstone complex as a reservoir series, is fully justifiable and reliable to use porosity determinations calculated from loggs.