Exploration Insights September 2019 - Page 18

18 | Halliburton Landmark and present-day TOC and HI is evaluated to determine at what level of maturity petroleum generation initiated. b. The maximum maturity at the base of the source rock is mapped and the immature, mature, or post-mature areas of the source rock are identified. c. A kerogen type map is constructed from analysis of geochemical data from immature samples. Depending on the inferred uniformity of the source rock, this can be a great guide to the original TOC and HI of mature or post-mature samples. d. Average TOC and HI values measured from mature and post-mature samples are restored to their original levels, following an empirical method based on the observed regression of these parameters across the oil window with maturation and generation (adapted from Cornford, 1998). It is assumed that the present-day TOC and HI measured from immature samples will be the same as the original values. If data are sparse, assumptions can be made from the inferred kerogen type and source rock analogues. Data are combined and orginal TOC and HI maps are created. e. The original TOC and HI values must be equal to or greater than the present- day TOC and HI values across the entirety of the mapped area. Spatial analysis of the maps is carried out and adjustment made, as required. Volume of Generated Hydrocarbons To calculate the volume of hydrocarbons generated, the method defined by Schmoker (1994) was selected, which uses the data discussed in the previous steps. The present- day and original masses of organic carbon in the source rock are calculated first; this combines Mass of organic carbon (M) = (TOC/100) x Density x Volume Units: M = g TOC, TOC = wt%, density = g/cm 3 , volume = cm 3 Total mass of hydrocarbons generated (HC) = -6 [(HI original x M original ) - (HI present day x M present day )] x 10 Units: HC = kg hydrocarbons, HI = mg HC/g TOC, M = g TOC Exploration Insights | 19 the source rock volume and TOC data with formation density. The mass of hydrocarbons that could still potentially be generated from the source rock is then calculated by multiplying the present-day mass of organic carbon with the present-day HI values across the map. This is subtracted from the ultimate mass of hydrocarbons that could potentially be generated (if the entirety of the source rock reaches its generative potential). Conversions from the mass of hydrocarbons to surface volumes of oil or gas per unit area can then be made. These calculated volumes should be taken as a minimum estimate. Volume of In-Place Hydrocarbons The volume of hydrocarbons generated is often huge compared with what is accumulated, as <10% of oil generated is typically preserved. The amount of in-place petroleum accumulated in the reservoir is the result of numerous factors, such as: expulsion efficiency; the timings of generation, trap and seal formation; migration; and other post-accumulation processes. The generation accumulation efficiency (GAE) is the ratio of the volume of petroleum in- place for the petroleum system to the total volume of petroleum generated (Magoon and Dow, 1994). GAEs from analogous known petroleum systems are applied to the volume of hydrocarbons generated, in a Monte Carlo-type simulation. This results in a range of volumes of petroleum in-place in commercial accumulations, with probabilities of occurrence. In highly effective unconventional resource plays any loss of hydrocarbons may be relatively negligible and, therefore, a high GAE applied; whereas, for conventional plays, GAEs may be very low, in some cases <1%, even for world-class source rocks (Magoon and Dow, 1994). THE IMPORTANCE OF CARRYING OUT VOLUMETRIC CALCULATIONS Petroleum System Ranking Systematic application of this workflow to petroleum systems in different basins will enable the quantitative comparison of source rock generative potential and petroleum systems effectiveness. This ranking will inherently take the thickness and lateral distribution of the source rock and the quantity and quality of the organic matter into consideration. This aids understanding of the factors that affect the estimation of volume of hydrocarbons generated and the subsurface risks associated with the generation of hydrocarbons. Additional Benefits The workflow produces standardized maps of measured and derived data that are insightful in themselves and can be incorporated into other workflows, such as source rock mapping, basin modeling, unconventional resource calculations, and play fairway mapping. The calculations applied in the workflow, particularly the restoration of the original TOC and HI, should be considered in any workflow requiring a source rock assessment. TESTING THE WORKFLOW Case Study The workflow was developed and tested on the Late Devonian to Early Mississipian Bakken Petroleum System, Williston Basin, central North America. The Bakken source rocks and the equivalent Exshaw/Banff Formation in Canada have been extensively studied and geochemically evaluated. The Williston Basin is a mature basin for conventional petroleum exploration. More recently, it has become synonymous with unconventional exploration due to the Middle Bakken resource play, which is sourced by the world-class Lower and Upper Bakken Shale Member source rocks. Data from over 780 wells across the Williston Basin were evaluated. The majority of the data fall in north-eastern Montana and western North Dakota as these coincide with the basin depocenter, the most heavily explored part of the basin (Figure 2). For the Lower and Upper Bakken source rocks, over 700 pieces of thickness data, 2,300 TOC results, 2,000 HI results and 1,900 pieces of maturity data (mainly Rock-Eval pyrolysis Tmax) were compiled, screened, and statistically reviewed. A constant formation density of 2.6 g/cm 3 was used. The map suite required for the calculation of the volume of hydrocarbons generated was created (examples shown in Figure 2). The type of organic material varies laterally across the basin in both the Lower and Upper Bakken, due to changes in the paleo depositional environment and the origin of the kerogen (Figure 2). Both source rocks predominantly comprise oil- prone, marine derived kerogen (Kerogen Type II), with gas-prone terrestrially derived kerogen (Kerogen Type III) in the northwest of the basin. This affects the generative potential of the source rocks across the basin; however, in the main depocenter, where the source rocks are mature to post-mature, the source rocks are principally Kerogen Type II. The Lower and Upper Bakken source rocks have average present-day TOC values of 13.6 and 14.2 wt%, and average present- day HI values of 305 and 316 mg HC/g TOC, respectively. Both source rocks are mature in the depocenter of the basin. Through restoration of the original source rock generative potential, it is predicted that the Data Inputs Isopach and Preservational (Areal) Extent Original Total Organic Carbon and Kerogen Type © 2019 Halliburton Thermal Maturity Figure 2 > Examples of the data used and maps created to calculate the volume of hydrocarbons generated from the Upper Bakken source rock.