New methods to reconstruct paleo-oil and gas compositions and P-T trapping conditions of hydrocarbon fluid inclusions in sedimentary basins

Abstract

Accurate predictions of fluid composition and P-T trapping condition of hydrocarbon fluid inclusions (HFIs) is crucial to understand complex oil and gas charging history and accumulation mechanism. However, traditional P-T prediction methods are not enough powerful to get reliable results because of difficulty in direct composition measurement and great uncertainties in indirect composition prediction for single HFI. In this study, we collected 296 crude oil and 983 gas condensate fluids to develop new composition and trapping pressure (Pt) prediction models. Based on the measured fluid composition, saturation pressure (corresponding to homogenization pressure, Ph) and volume data, we firstly calibrate the Ph, the degree of bubble filling (Fv, at 20 °C) and isochores predictions for the measured fluids through matching saturation pressure and volume calibration and then established a standard fluid database with fluid composition, Ph, Fv and Pt. For the first time, a new hydrocarbon fluid composition prediction model was developed which is only function of Fv and homogenization temperature (Th) of HFIs. The new composition model allows directly predicting the entire fluid compositions from methane (C1) to heptane plus (C7+) fractions in HFI based on Fv and Th. Furthermore, we developed different Pt prediction models for varying fluid compositions and phase states of HFIs. For oil-bearing fluid inclusions which homogenize into liquid phase (OFIL) methane molar content is the only variable that is needed to predict Pt with a high degree of accuracy (8.2–12.3% for average absolute deviation, AAD). However, for gas condensate-bearing fluid inclusions (GCFI), more composition (C1–C7+) together with the physical property (molecular weight and specific gravity) of C7+ composition must be incorporated into the prediction model to obtain a better prediction accuracy of Pt. The new Pt prediction models give prediction accuracy of 11.2–17.6% (AAD) for GCFIL (homogenizing into liquid phase), and 15.9–22.8% (AAD) for GCFIG (homogenizing into gas phase). Our new composition and Pt prediction models can give the best and fastest predictions of hydrocarbon fluid composition and trapping condition and are of great significance for petroleum charging history reconstruction in sedimentary basins.