Comparison between laboratory and petrophysical-derived methods to identify the clay minerals in an eastern Kopet-Dagh gas field

Shurijeh sandstone formation (early Cretaceous) is the main and subsidiary reservoir rock in the eastern Kopet-Dagh gas fields which consists of shale interlayer in its stratigraphic sequence. Each clay mineral with any distribution pattern have a great impact on the response of some logs such as resistivity, sonic, density and also affect the porosity and permeability calculations. It also plays an important role in the sandstone reservoir evaluation. In this research, clay mineralogy of Shurijeh reservoir Formation (D1 division) and their distribution patterns in two wells located at eastern Kopet-Dagh field were identified using instrumental analysis such as X-Ray diffraction (XRD), X-Ray fluorescence (XRF), scanning electron microscope (SEM/EDX) as well as applying five petro-physical cross plot. Results showed clay typing only based on petro-physical cross plots can produce incorrect or controversial results and should be completed by core analysis data. Based on the laboratory analysis results, clay minerals are kaolinite, magnesium-rich chlorite, illite and in some cases glauconite. Distribution pattern in producing well is structural and dispersed (pore-filling and pore lining) and in non-producing well is structural, dispersed (pore-filling, pore lining and pore bridging) and laminar. The amount of autogenic dispersed clay minerals is low in producing well and high in non-producing well and in some cases reaches to a limit in which the formation lose its reservoir quality.