Abstract

The Zan Zan Field in eastern Irion County, Texas produces from a 20 to 50 feet thick basinal sandstone which occurs between Wolfcampian and Strawn limestones. Water saturation calculations, based on crossplotted density-neutron porosities and dual induction resistivities are rarely less than 50%. Water production from typical wells in this field is 5-6% of the oil production, which had cumalitive production of 644,549 barrels of oil and 1.4 BCF gas as of November 1991.

Thin sections from this sand exhibit a complex lithology. Argillaceous rock fragments are approximately 15% of the detrital constituents. Patches of kaolinite with intercrystalline micropores are common although not abundant. Secondary pores occur as both oversized mesopores and intraparticle micropores. Grain rimming siderite is very well developed in most thin sections observed.

The composition and fabric of this sand makes it difficult to predict the producing character of wells from well logs. Neutron porosities are anomalously high as a result of clays in rock fragments and shale laminations. Where the volume of siderite is significant, density logs may give eroneously low porosities, but typically the total porosity calculated by the density log will include the ineffective microporosity which is common. The occurrence of siderite coating sand grains may also be effecting the electrical properties of this rock. The path of greatest conductance measured by the dual induction log may be along this continuous iron carbonate coating along the pore walls. The lack of significant water production at Zan Zan is either a result of a high proportion of water in microporosity associated with clay minerals and secondary porosity, and/or Sw calculations are incorrect due to siderite grain coatings.