Innovative Methods for Quantifying Poorly Known Parameters* Necessary for Numerical Basin Modeling By

Deterministic models, in which mathematical equations are used to predict the occurrence of various phenomena in the subsurface, are widely used today in basin modeling. However, these models suffer from a number of weaknesses, including incorrect or incomplete equations, incorrect constants in the equations, and errors in input variables. Several alterna tivesto present deterministic basin models are illustrated here by showing their application in describing permeability. Self similarity, the basis of fractal theory, states that many patterns observed on one scale are also present on other scales. Measured permeability data from hand samples of re servoirrocks must be scaled to adequately describe effective formation permeabilities for carrier beds. Self similarity may also be valuable in evaluating the contribution of fracture permeability to fluid flow. A self-similarity approach may be useful even where self similari tyis not strictly valid. A new method for predicting the occurrence of overpressuring and the volume of fluid ex pelledfrom both normally pressured and overpressured rocks is based on acquisition of empirical data on the maximum sedimentation rates at which normal pressuring can be maintained under well-constrained conditions. The parameters required to use this method (original and present porosity, viscosity of water, compressibility, present thickness, and recent sedimentation rate) can be obtained more accurately from empirical data than the quantities required for calculation of fluid flow from fine-grained rocks using Darcy's law. Tensional fractures can play a dominant role controlling fluid flow in rocks with low ma trixpermeabilities. The occurrence and intensity of tensile fracturing can be predicted from stress-strain considerations. It may also prove possible to estimate the degree of fracturing by applying self-similarity concepts to fault patterns observed from seismic data, or from empirical data on the occurrence of overpressuring. Great differences exist between human time scales, which govern reservoir engineering, and geologic time scales, which govern fluid movement in basins. Differences that are of acute importance to a reservoir engineer are often of no practical importance for basin modeling. In basin modeling it is most important to specify permeabilities of seals and of fine-grained rocks as they expell water and hydrocarbons. It is also important to determine the minimum permeabilities capable of permitting long-distance lateral fluid flow. Obtaining reliable data for such rocks should be a main objective of research on basin modeling. * 平 成2年6月14日 第55回 石 油 技 術 協 会 総 会 地 質 ・探 鉱 部 門 シ ン ポ ジ ウ ム 「堆 積 盆 地 評 価 法 とそ の 応 用」 に て講 演 ** Technology Research Center , Japan National Oil Corporation, 1-2-2 Hamada, Chiba 260, Japan (石 油 公 団 石 油 開 発 技