What Is the Lithology of Reservoir Rock in Jackson - Assignment Example

There are similar results in Bulk density vs. PEF cross-plot and the cross-plot of Rho_maa vs. U_maa when viewed in the context of lithology. However, the distribution and availability of coal in the formation base are more evident in the Bulk density vs. PEF cross-plot analysis. This difference in the dispersal and availability of coal might be attributable to the fact that in the cross-plot analysis of Bulk density vs. PEF, rock composition is identified by utilizing a combination of two minerals, as opposed to cross-plot analysis of Rho_maa vs.

U_maa wherein three minerals are used in order to pinpoint the rock sequence or its lithology. In the case of Bulk density vs. PEF cross-plot, mineral lines need to be construed and evaluated based on the lithology of the area so that the rock description and classification of the formation will be properly identified. On the other hand, Rho_maa vs. U_maa cross plots result in accuracy is heavily influenced by matrix choice, which needs to be the right one in order to provide exact results. In addition, both of the cross-plots gave poor results in situations of the existence of a rough hole or heavy drilling mud.

• Discuss the effect of saturation, shale content, and other parameters on cross-plot accuracy and lithology identificationHydrocarbon saturation of gas or light hydrocarbons triggers an increase in the apparent porosity from the density log. In other words, bulk density decreases along with the porosity from the neutron log. It also causes an upward shift on the neutron-density cross-plot. The shifting that occurs starts from a liquid-filled point with similar porosity going the left, parallel to the isoporosity lines.

However, if the application of gas correction is inaccurate, this would result in a low porosity reading, as well as a major error in rock sequence indication or lithology. The shale content will cause an alteration of the shale point on neutron-density and sonic-density cross-plots from the southeast quadrant to the northeast quadrant and the lower center of density-photoelectric cross-section cross-plot. Approximation of shale values on material parameters of shale that might exist in permeable beds is possible though.

Another aspect that can influence the accuracy of cross-plots and proper identification of lithology of formations is secondary porosity. Secondary porosity happens as a result of sonic logs ignoring buggy porosity and fractures while responding to intergranular porosity. Neuron and density tools, however, respond to total porosity. What happens is cross plots with sonic logs involved transfer points from correct the lithology line and will show values less than the actual total porosity. In the case of neutron-density cross-plots, however, total porosity values are conclusive and accurate, just by basing on the charts.