MONTE CARLO TECHNIQUE APPLIED TO THE STUDY OF SAND/SHALE IN A GEOHYDROLOGICAL SECTION

Abstract

A geohydrolvgic l erection is a representation of the subsurface, depicting clearly the location of Previous and impervious horizons. In order to make mathematical analysts easier, certain assumptions are made regarding tho layers that comprise the subsurface. It is ae.:umod that the layers are horizontal, homogeneour~, isotropic and of infinite lateral extent. Purthermore, it is also ao umed that the last layer extends to infinity In the vertical direction. Other assumptions are also made depending on the prcbler at hand. If there is 6 marked contrast in the electrical properties ( resistivity) at the interfaces, then the uses model may be roprocented by a geoclectric section. The concept of the ;oaelcctric section is essential for analysis of data acquired by gecelectri cal survey for ground water. Ground water is known to occur in unconsolidated cedimonts, consolidated sediments and in non-sedimontary roi In areas which are prodominontly sedimentary, an alternate sequence of sand and shale is commonly encountered. Hero // shale refers to impervious hor .Wore like clay, shale etc. with the exception of carbonate rocks. Similarly, the term sand has been intend©d for all those sedimentary horizina which are permeable, can yield sufficient ground eater to be alasoifie:l as aquiforsand to which a primary porosity may be attributed. This includes variations of sand, sandstone, gravel, silt etc. Aquifers can bo unconfined or confined (artooian), depending on the abocenao or presence of a confining layer. they are also loosely termed as pay horizons. Information regarding the pay horizons can be had from sand/shale ratio studies. 1.2 SAND/STALE RATIO U' U DI ES Sand/shale ooquonces are one of the various lithologtde whit ay be described by statistical models. 'Sedimentation models summarize the characteriotico of strata deposited in a contain env=ironment and predict the anticipated thicknoso and continuity of various lithologies, the typo and rate of lateral change, and the nature of contactE ~SiNJttAL 1q? G ). Thus nand/ohaio ratio can be used as a potrorful parameter in deciding they possible locations of pay horizons# ouch information is quite useful in deciding drilling strategy. w Attempts at inferring stratigrephy from seismic date, have yielded encouraging results. The techniques adopted can be classified w deterministic and stochastic. !4ATBI and RICE (1969) chose a statistical approach in discriminatory analysis for the determination of variation in stratigraphic condition. The parameters they selected for discrimination were taken from seismic traces. Their technique was not u iiverealiy successful....