Using Isotopic Analysis Technique in Complex Subsalt Formation

Overview

An operator requested Halliburton Sperry Drilling provide GasFact™ data logging services for real-time well prognosis in the Gulf of Mexico (GOM) using stable carbon isotope ratio analysis. While exiting the base of salt, displaced cretaceous carbonates with a thermo-genetically mature carbon isotope signature were positioned over younger Miocene sandstones with an immature biogenic carbon isotope signature. These formations were followed by a shaly section with wet stringers and then the thermogenic mature reservoir. The Surface Data Logging (SDL) team recommended an isotopic gas analyzer paired with a flowline constant volume extractor (CVE), in combination with standard total hydrocarbon analysis (THA) and C1-C5 gas chromatography.

Challenges

Reducing uncertainty in real time is necessary for potential hazard identification during drilling and reservoir characterization. The challenge in this case was to determine fluid origin while drilling through the following complex sections:

• Subsalt nonconforming cretaceous carbonate and pre-miocene marl and clastic
• Transition zone from cretaceous carbonate to in-place sections—a mixture of carbonate and shale at the top to a more shaly section at bottom
• Mid-miocene sand-oil bearing on exploration wells (expected to be wet below OWC)
• Shaly section with sand stringers and possible losses
• Reservoir-oil-bearing sand with possible shaly/sandy intervals

Data quality assurance was another concern, particularly when identifying drill-bit metamorphism, which can help optimize drilling activity and constrain gas data quality. The generation of ethylene and propylene gas caused by drill-bit metamorphism indicates high-temperature-induced cracking of drilling and formation fluid, which can affect the natural gas signature of formation fluid and make accurate analysis and interpretation difficult.

Solution

Global GasFact service support was provided for real-time, accurate carbon isotope data analysis of methane, ethane, and propane. The SDL team provided necessary hands-on procedural steps related to the CVE and GasFact analyzers at the wellsite. The operator’s petrophysics team provided a well prognosis.The GasFact service used the Baseline 8900 Gas Chromatograph (GC), Baseline 9000 Total Hydrocarbon Analyzers (THAs), and Compact Science Systems’ IsoLogger Mass Spectrometer (GC-C-IRMS) at the rigsite to provide analysis of a flowline gas-in-mud sample.The SDL team members monitored the GC for the presence of extra gas peaks between ethane and propane (ethylene peak) and between propane and isobutane (propylene peak) for drill-bit metamorphism identification.

Results

The GasFact service-generated data analysis successfully identified different fluid type origins during the transition from older displaced Cretaceous carbonate overlying the younger Miocene formation—a thermogenic to biogenic signature—confirming the operator’s well prognosis expectations.