The Technology of Distinguishing Condensate Gas and Light Oil Bearing Zones in Low Permeability Field
Authors: Xiao Yuxiang , Li Wei , Liao Mingshu , Wu Lei 
Published: December 2007

Abstract
Many low permeability fields with condensate gas and light oil having beendiscovered in the world range. But it is very difficult to distinguish gas andoil-bearing zones by well logging evaluation technology because of the smallresponse difference between the gas and oil in such oil fields. Consequantly, much more well tests have to be carried on which results in extra cost, andcertain terrible lost that can not be made up for may conducted by theimperfect strategy according to the wrong distingushing results sometimes. The"MCM" (Multiwell log Curve Micofaces) technology of distinguishing condensategas and light oil bearing zones in low permeability field is suggested in thispaper after the general theoretical analysis. The special technique of "MCM" isto determine the GOC (oil-gas contact) by individual well vertical correlationand multi-well comprehensive correlation based on optimizing conventional logcurve combination and investigating the micro-faces of these curves. All GOCwere accurately determined after the special technology was applied in a such atypical oil field in west of China and seventeen wells were interpreted, whichwere supported by all test and production data. The "MCM" technology played ainstructive role in the later development planning of the oil field, whichshows it has significant application prospect in the future.

Introduction
Though modern well logging technology goes ahead vigorourly and many kindsof imaging tools and interpretation software come into use throughout theworld, the solving of some problems is not apparently making good progress, such as the distinguishing of conensate gas and light oil bearing zones in lowpermeability reservoir still remains to be a hang-up place to the people. Suchzones are so difficult to distinguish because of:Low log response of gas and oilThe most log response comes from the rock matrix rather than the pore fluidbecause the matrix holds large proportion of the rock volume in the lowpermeability reservoir. Compared with that of the high permeability formation, the mud filtrate invasion in the low permeability one is relatively deepbecause it needs much more large range of the borehole formation to keep thesame pore volume for the filtrate when the mud cake appears during the courseof drilling, therefore the log response of gas and oil is further reduced.Small log response difference between the gas and oil
The difference of the density between the condensate gas and the light oilin the reservoir condition because the condensate gas is heavier than the dryone as well as the light oil is lighter than the black one. Besides this, thechemical composition of the condensate gas and the light oil is much moresimilar to each other than the dry gas and black oil. Therefore the logresponse difference between the condendate gas and light oil decreases.
For all these reasons as mentioned above, the satisfactory result can not beacquired when the methods are applied to distinguish the condcusate gas and thelight oil bearing zones in spite they are suitable for distinguishing those inthe normal situation.
The MCM Distinguishing Technology

Basic Theory
The basic theory of the MCM technology is the same as that of the generalmethods of distinguishing gas and oil bearing zones. It makes good use of thelog response characteristics of gas bearing zones as follows:RT: RT increases. In general, the irreducible water saturation of gasbearing zone is lower than that of the oil bearing zone in the same reservoircondition, therefore the resistivity of gas bearing zone is comparativelyhigh.CN: CN decreases. The neutron porosity will decrease because of the lowerhydrogen index when there is certain gas content in the pore fluid of theformation.DEN: DEN decreases. The density log value will decrease for the lower porefluid density while pore fluid contents gas.

Basic Theory
The basic theory of the MCM technology is the same as that of the generalmethods of distinguishing gas and oil bearing zones. It makes good use of thelog response characteristics of gas bearing zones as follows:RT: RT increases. In general, the irreducible water saturation of gasbearing zone is lower than that of the oil bearing zone in the same reservoircondition, therefore the resistivity of gas bearing zone is comparativelyhigh.CN: CN decreases. The neutron porosity will decrease because of the lowerhydrogen index when there is certain gas content in the pore fluid of theformation.DEN: DEN decreases. The density log value will decrease for the lower porefluid density while pore fluid contents gas.