Rock-Eval hydrogen index (HI) is often used to compare relative maturities of a source horizon across a basin. Usually, there are several measurements from the source horizon at a single well, and the mean hydrogen index is calculated, or the S2 is plotted against TOC. The slope of the best fit line through that data is used as the representative HI for that well (sometimes referred to as the ‘slope HI ’ methodology). There is a potential flaw in both these methodologies; however, that renders the calculated HI as misleading if the source horizon being examined is not relatively uniform in source quality, vertically in the stratigraphic column. From a geologic perspective, it would be unusual for the source rock quality not to vary vertically in the stratigraphic column. Organic matter input, preservation, dilution, and sediment accumulation rate typically vary in many depositional environments over the millions of years required to create a thick source rock package. Nevertheless, there are source rocks which do display remarkable source-quality uniformity from top to bottom of the stratigraphic package. We have examined source rocks from several basins where the source quality is relatively uniform over the stratigraphic column, and source rocks where the source quality varies greatly over the stratigraphic column. Methodologies to assess hydrogen index at specific wells for the se two scenarios differ. Most geoscientists may not be familiar with why a single technique is not suitable for both these scenarios, or how to correctly use hydrogen index as a relative maturation proxy in the case where source rock quality is not uniform. We will demonstrate how to determine if your source rock quality is uniform or varied relative to HI over the stratigraphic column, and how to assign a hydrogen index to the different source facies when that source rock quality is not uniform. Further we will illustrate how to estimate the original hydrogen index of the different source facies and assign each a transformation ratio. The transformation ratio is a better proxy for relative maturity, since different source facies may have different present-day hydrogen indices, but their present-day transformation ratio should be quite similar.

Jean Hsieh, Normand Begin, Maz Qayyum, Talisman Energy Inc., part of the Repsol Group

The Kurdamir-Topkhana field of Kurdistan is a carbonate oil and gas reservoir that was deposited in a passive margin ramp setting during the Oligocene. It is dominated by alternations of grainstone/boundstone/rudstone and packstone/wackestone beds. Reflux dolomitization early in the Miocene has noticeably increased, in places, both the porosity and connectivity of at least the Upper reservoir interval, as interpreted from seismic inversion data. The Lower reservoir interval was less affected by dolomitization and exhibits good porosity development within individual clinoforms, but much poorer connectivity between clinoforms. The entire reservoir interval is located within two anticlinal four-way closures formed above the roof thrust of an anti-formal stack developed within deeper Mesozoic carbonates; much of the observed fracture network is a result of this deformation. Modelling this complex reservoir presents significant challenges, not least because of the interdependency of many of the reservoir’s key characteristics.

The depositional and diagenetic history of the reservoir was determined through the study of several cores cut from the wells drilled in the field along with an understanding of the regional setting. A 3D seismic survey was inverted for acoustic impedance and the presence of gas-filled porosity can be imaged through the AI volume. Combining the understanding gleaned from the core and seismic data with the fracture data from borehole image logs, a detailed reservoir model was built. The strategy and conceptual models used for building this geostatistical model will be presented, along with our interim products and the lessons we have learned thus far on combining data of different scales.