Seismic Characterization of Sandstone Diagenetic Facies: A Case Study

In Qijia area, Songliao basin, China, lacustrine sandstones in Cretaceous Qingshankou Formation are major targets for exploring hydrocarbons. The formation is composed of multiple high-frequency sequences with complex but predictable stack patterns of LST and HST fluvial-dominated deltaic systems. Sandstone diagenetic facies has significant influence on reservoir quality. Between the two major diagenetic facies, calcareous facies is characterized by calcite cement in pore, and clayey facies is dominated by kaolinite/illite cement in pore. With a similar porosity, clayey facies has been measured with higher permeability than calcareous facies does. As a result, clayey facies is of better reservoir quality.

In core observation, thicker distributary sandstones are interbedded with thinner delta-front sandstone sheets, lake shales, and less frequently, thin limestones and marls. Wireline logs reveal a wide range of acoustic velocities in rocks. Limestone and marl has the highest velocity; moderate velocity is measured in calcareous sandstone; and clayey sandstone is of lowest velocity. Shale velocity varies from high in shallower water facies to low in deeper water facies.

Seismic interpretation of diagenetic facies requires first to tie the two diagenetic facies to proper seismic mapping units. This was realized by a frequency adjustment of seismic data that links seismic events to velocity zones and systems tracts. Stratal slices of different systems tracts revealed detailed polarity/amplitude/impedance distribution for various depositional facies in a quantitative sense, which in turn could be interpreted in terms of diagenetic facies. An initial evaluation indicated that the seismic diagenetic facies maps have a decent correlation to reservoir quality and test results in wells.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California