Leached Secondary Porosity by Meteoric Water in the Upper Triassic Yanchang Formation of the Ordos Basin, China

Lan, Yefang; Huang, Sijing; Huang, Keke

Yanchang sandstone, mainly composed of arkose and lithic arkose, is beneath the regional unconformity at the base of the Jurassic section. Abundant oil resources are present in the Upper Triassic Yanchang Formation, and secondary porosity is the most important reservoir space. Based on the information of petrographic investigation, scanning electron microscope (SEM) and a lot of statistics of data, we present an example of leaching by meteoric water in the Yanchang Sandstone of Triassic in Ordos Basin.

The interpretation that the generation of secondary porosity is greatly influenced by meteoric water by three reasons: (1) The upper part of Yanchang sandstone, with a closer distance to the unconformity, has better reservoir properties, averaging porosity of 16% and permeability of 5md; whereas sandstones from deeper and farther away from the unconformity show lower values of porosity (0.2-9.3%) and permeability (lower than 1md), and thus relatively poor reservoir quality. (2) More dissolution pores formed by the dissolution of feldspar and lithic grains are observed in the upper part of the Yanchang Formation, resulting in the increase of the porosity of about 5%. Kaolinite is abundant in the upper part sandstone but with a smaller content in the lower part. Subsequent detailed petrographic study and XRD analysis confirmed that the potassium depletion had been caused by potassium feldspar dissolution, resulting in precipitation of kaolinite (commonly filling in the pores) and creation of secondary porosity. (3) The study of typical well also shows the characteristics that the upper members closer to the unconformity display lower feldspar content, corresponding higher kaolinite content, higher thin section porosity, more secondary porosity dissolved by feldspar, and also higher measuring porosity and permeability.

The above evidences indicate that the upper members have undergone relatively intense alternation of potassium feldspar to kaolinite, with the resulting loss of bulk potassium content, higher secondary porosity and increasing of kaolinite. This could be explained as a result of subaerial weathering of the top of the sandstone body, and leaching by meteoric water controlled the secondary porosity development. The prediction model of reservoir quality should be concerned by means of this mechanism of generation of the secondary porosity.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013