Real-Time Fluid Characterization Using Advanced Gas Analysis in Hith and Gotnia Formations

Abstract

The Hith and Gotnia Formations in Kuwait were considered to only be acting as seals for the early Jurassic hydrocarbon bearing reservoirs. However, recent hydrocarbon discovery within the Gotnia Formation has proven that they have reservoir potential as well. Due to High Pressure-High Temperature (HPHT) conditions of these formations, casing is set immediately after drilling, thus limiting open hole data acquisition. To overcome this problem we deployed a single mass spectrometry service for advanced gas analysis to understand and characterize the fluid variations within the Hith and Gotnia Formations. Two mass spectrometers, for gas in and gas out detection, were connected to constant volume degassers in the suction pit and shaker house, respectively. First, the dynamic oil based mud (OBM) discriminator differentiated OBM contamination and subsequently the unique ‘real-time’ algorithms quantified concentrations of formation hydrocarbons. The formations under evaluation were divided into 3 major geochemical zones based on acquired gas data. These geochemical zones were marked by changes in mud gas response and were closely linked to lithological properties, each lithology type having a distinct gas signature. The Hith Formation consists of thinly interbedded anhydrite and limestone and comprises the first geochemical zone. With low gas response, gas ratios point to a potential oil fluid type although the formation is likely unproductive due to low permeability and high water content. The four halite intervals within the Gotnia Formation were grouped together as the second geochemical zone, having no production potential due to their low porosity and permeability. They behave as a seal to the hydrocarbons in the interbedded anhydrite-limestone intervals. The three anhydrite plus limestone intervals within the Gotnia Formation belong to the third geochemical zone. Comparison of the oil signatures in these intervals revealed a decrease in fluid density from top to bottom. The different fluid density within each interval and the sealing efficiency of the interbedded halite suggest that there is no direct fluid communication between the individual anhydrite cycles. This paper covers in details the response and interpretation of the gas signatures in each zone.