A Real-Time Method to Identify Brittle Zones in Carbonate-Rich Mudrocks using Bulk and Trace Element Geochemistry:A Study in the Eagle Ford,Haynesville,and Niobrara Formations

摘要

With advancements in technology such as horizontal drilling and hydraulic fracking,operators are able to pursue reserves in unconventional mudrock reservoirs.Brittleness,one of the many pre-screening considerations,is an important parameter because it determines whether a mudrock can be effectively stimulated via hydraulic fracking.The industry currently uses several geochemical signals(e.g.Si/Al and Si/Zr)to identify authigenic silica phases present in an unconventional reservoir.Cemented horizons are prime candidates for placing hydraulic fracks due to the strengthening effects of mineral cements on the rock frame.A similar geochemical method for readily indicating the occurrence of authigenic carbonate has not been identified.This study documents trace element geochemical differences between biogenic(detrital)carbonate phases and associated cements so that chemical proxies may be used to differentiate authigenic carbonate phases using bulk geochemical data.Both carbonate-rich formations(e.g.Eagle Ford and Niobrara)and argillaceous formations(e.g.Haynesville and Marcellus)are examined to gain insight into reservoir brittleness,using bulk and trace elements such as Ba,Mg,Mn,Fe,Sr,and Ca.The goal is to develop a technique that can be implemented real-time by the mudlogging unit at the wellsite and during the initial core analysis phase.This method will allow a more targeted placement of hydraulic fracking zones to increase permeability and hydrocarbon production in mudrock reservoirs.Electron probe micro analysis(EPMA)on several types of carbonate was conducted on low(0.45 %Ro)and high(2.5 %Ro)thermal maturity Eagle Ford and Haynesville Formation samples,respectively.The EPMA reveals that Sr is the primary elemental signature of the authigenic carbonate phase within the low maturity Eagle Ford.The Haynesville EPMA reveals higher variability of Fe,Mn,Sr,Mg,and Ba trace element concentrations,however the dominant elemental signature associated with the authigenic phases is elevated concentrations of Fe and Mn.Utilizing XRF,Sr/Ca and Ca-Fe-Mg cross-plots can be used as proxies to identify authigenic carbonate in the Eagle Ford and Haynesville Formations respectively,and can be used to indicate brittle zones for target adjustments at the wellsite.