The reservoir quality (RQ) of unconventional resources—defined as the ability to produce hydrocarbons economically after hydraulic fracture stimulation—is influenced by the amount and type of organic matter present in the formation. Types of organic matter include kerogen (solid and insoluble organic matter, similar to coal), bitumen (soluble but immobile organic matter, similar to shoe polish), and oil and gas (potentially producible organic fluids). In gas-bearing shales, gas is typically stored in and transported through pores hosted by kerogen. Thus, a high quantity of kerogen typically leads to higher porosity/permeability and is considered a positive RQ indicator in gas shales. In oil-bearing shales, mineral-hosted pores contribute more significantly to hydrocarbon storage and transport, while kerogen and bitumen can trap fluids by adsorption, clog pore throats, and swell. Thus, a high quantity of kerogen and bitumen can be considered a negative RQ indicator in tight oil plays. At the same time, the quantity of oil is a positive RQ indicator, emphasizing the value of differentiating the types of organic matter in tight oil formation evaluation. Previously, attempts have been made to assess tight oil RQ by separating the quantity of oil from the quantity of kerogen and bitumen based on cuttings analysis. However, properties of cuttings, particularly the quantities of contained fluids, are not necessarily representative of the formation because a fraction of the fluids typically escapes from the cuttings before they arrive at the shale shaker.
Here we present an index for assessing tight oil RQ using exclusively downhole logs, ensuring representativeness. The index is named the reservoir producibility index (RPI), and it treats oil as a positive RQ indicator but kerogen and bitumen as negative RQ indicators. In computing the RPI, the quantity of oil is measured primarily by NMR logs while the quantity of all organic matter is measured by nuclear spectroscopy logs. The utility of the RPI is demonstrated in examples from several tight-oil plays, in which zones identified by the RPI as having the highest RQ are found from production data to be the most productive.
