Openhole Crossplot Concepts A Powerful Technique in Well Log Analysis
Authors: Fertl Walter H. 
Published: December 2007

A multitude of crossplot concepts for well logging parameters is available to the formation evaluation specialist. Raw and/or calculated logging parameters can be crossplotted on linear, semilogarithmic, and other exponential scales. Input data include resistivity, acoustic, and nuclear logging measurements and, if available, core, test, and production data. Applications of specific crossplot concepts allow recognition of log calibration problems, normalization of basic log measurements, and determination of lithologic reservoir characteristics in clastic and complex mineralogy. Additional information includes primary and total porosity, formation-water salinity, distinction between oil and gas, estimation of reservoir grain sizes, prediction of irreducible water saturation and, hence, anticipated water cut, heavy mineral analysis, and other data. This discussion focuses on advantages and limitations experienced with numerous crossplot techniques. Frequent reference is made to formation evaluation problems encountered in the North Sea area.

Introduction

Crossplotting of porosity logging data has been used since the early 1960's.1–4 Today, an extremely large variety of two- and three-dimensional crossplots (z plots) is available. While computer processing allows quick and easy data handling, hand plotting still provides an effective check for the experienced log analyst.

Monomineral Porosity Model

Assuming one studies a reservoir rock of known lithology which is clean and/or shale corrected, then each porosity can be expressed. as follows5,6:b, fN= function of [rock matrix, porosity, amount and type of fluid in pore space (mud filtrate, oil, gas)] .t=function of (rock matrix, porosity, amount and type of fluid in pore space, compaction, secondary porosity).

Mental recollection of these and other basic log response functions greatly assists in understanding, defining, applying, and evaluating the potential of the numerous crossplotting techniques.

Binary (Two-Mineral) Porosity Model

Complex Reservoir Rocks

Two basic mineral constituents are present. The mathematical solution requires, in addition to the unity equation, only two of the three basic porosity response functions listed as follows. Ambiguity exists in certain crossplots due to nonlinear dolomite response of neutron-type logs.b, fN=function of (type and bulk volume percentage of rock matrix components, porosity, fluid in pore space, shale).t=function of (type and bulk volume percentage of rock matrix components, porosity, fluid in pore space, shale, secondary porosity).1.0=porosity+S(bulk volume matrix components).

Complex Reservoir Rocks

Two basic mineral constituents are present. The mathematical solution requires, in addition to the unity equation, only two of the three basic porosity response functions listed as follows. Ambiguity exists in certain crossplots due to nonlinear dolomite response of neutron-type logs.b, fN=function of (type and bulk volume percentage of rock matrix components, porosity, fluid in pore space, shale).t=function of (type and bulk volume percentage of rock matrix components, porosity, fluid in pore space, shale, secondary porosity).1.0=porosity+S(bulk volume matrix components).