Evaluation of the Petroleum Potential in the Wushi Field, P. R. China

J-B Peyaud¹, Jun Cai², Shu Sheng Guo², and Yong De Gao^2
1Baker Hughes, Perth, Australia
²CNOOC, Zhanjiang, China

Hydrocarbon production directly from shales, i.e. the source rocks, has completely changed the gas market in the US. As a consequence, these formations, which were considered as an element of the petroleum system (the source or the seal), are now regarded as potential producers and get a growing attention around the world. In order to evaluate the potential of a shale, it is necessary to characterize the occurrence of natural fractures, the mineralogy, the organic matter (amount, type, maturity, sorption capacity), the rock mechanical properties (brittleness, stress regime, stress orientation and intensity, pore pressure), as well as the porosity and the saturation (volume of free hydrocarbons), focusing on variations: the key is to understand the distribution of heterogeneities, either to target them for fracture stimulation or to avoid them as barriers. As it was demonstrated in the US, a relation exists between the mineralogical composition and the mechanical properties: the brittleness of the formation tends to vary as the amount of quartz. Conversely, the potentially more productive intervals are those with higher organic content as these combine free hydrocarbons present in the pore network with hydrocarbons adsorbed onto the organic matter.

The productivity of the well depends then to our capability to initiate fracture stimulation in brittle intervals located close to organic-rich intervals. Based on the relation between quartz and brittleness, mineralogical information is key to locating brittle intervals, organic-rich intervals and potential barriers. In this communication, we shall present how the mineralogical information can be used to locate intervals of interest through rock typing and evaluation of the total organic carbon (TOC). The TOC calculated from the mineralogical log are then compared to a petrophysical evaluation made using the Delta LogR method.

The target formation is a clastic series comprising sandstones and shales in the Wushi Field, People’s Republic of China (PRC). It was logged with a comprehensive suite of logs including resistivity, neutron, density, acoustic properties, spectral gamma, spectroscopic mineralogy and borehole imagery. The logging was carried out by Baker Hughes, using their tools. As Baker’s Formation Lithology Explorer (FLeX) has the capability to measure directly the carbon in the formation, a direct measurement of TOC was available. It was compared to a petrophysical evaluation and laboratory measurements.

The target shale is located between 180 and 300 m in the log displayed in Fig. 1. It presents 2 parts: the upper one (180-240 m), characterized by a high content in feldspars, and the lower one (240-300 m) containing less feldspars. The amount of organic matter remains low in the sandstones (1% or less) and associated shales except between 110 and 120 m where the shale layer contains up to 6% TOC. In the shale, the TOC varies between 0 and 6% with an average of 2%, except between 215 and 235 m where TOC is lower than 2%. This interval also contains less clay minerals than the rest of the shale, with meter thick sand layers identified. The TOC estimation based on the Delta LogR method yielded similar results for the shale, an average TOC of about 2% and a maximum of about 5%. Results were also consistent with TOC measured in the laboratory. Considering the temperature at the bottom of the well (about 145 C), it is expected that the organic matter is mature enough to generate gas.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013