A New Approach for Outcrop Characterization and Geostatistical Analysis  of Meandering Channels Sandbodies within a Delta Plain Setting using Digital Outcrop Models: Upper Triassic Yanchang tight sandstone Formation, Yanhe outcrop, Ordos Basin

Key words: 
• digital outcrop models /
• reservoir morphology /
• delta plain

Abstract: Digital outcrop modelling is a method of imaging outcrop as a series of polygons viewable with 3D visualization technology, on to which digital geological information (e.g., outcrop sections) may be stacked. The technique achieves a digital outcrop model (DOM) from which comprehensive measurements and interpretation may be undertaken. A DOM is able to incorporate 3D data from ground-based laser scanner, remote-sensing imagery, high precision grayscale photographs, ground-penetrating radar and other all geo-digital sources. The use of DOM in combination with traditional sedimentological field data helped improve the accuracy and efficiency of meandering delta plain sand body characterisation and consequently also improved the qualitative and quantitative assessment of data for the purpose of reservoir modelling.
      We used the DOM to measure and precisely describe the distribution characteristics of channel sands of the Quliu delta plain in the Ordos Basin. In order to provide the basis for a comparative reservoir model, the 3D distribution model can be quality controlled through the measurement of the characteristics of channel sand reservoir in varied sites and predicting the morphological characteristics of ground sand in the outcrop area by groundpenetrating radar. The DOM shows: (1) Tanjiahe section channel forms; these sandstone bodies form elongate lenses with a flat upper surface, a width of 192 m and a maximum thickness of about 14 m of the most biggest channel body sand. The boundary between channel and surrounding rocks is clear.Four stages of deposition are identified within the sandstone bodies indicating a complex positive rhythm. (2) Branch channels to the meandering river are isolated on the delta plain. These sandstone bodies show lateral migration and vertical stacking which is accurately described in the 3D DOM. The horizontal width of the multi-period superimposed channels of the sandstone bodies are 192.4 m, 32.54 m, 41.83 m and 79.97 m, thickness are 14 m, 4.52 m, 3.85 m and 5.66 m giving a width to thickness ratio of 12:1. (3) Through analysis of typical reservoir performance of the sand bodies encountered, reservoir properties can be assigned to different parts of the DOM. The delta plain branch channel sandstones are classified as poor quality. In the main channel sands the highest quality reservoir would be found in the lower part of the channel which then decreases upward. The lowest quality reservoir is found in the two channel wings. Reservoir quality is best at the thickest part of the sandstone body in every period channel where it achieves a porosity of 8% and a permeability of 0.13 mD. Building of a DOM integrated with geo-data is an effective method to quantitatively analyse outcrop analogues including the 3D spatial characteristics of potential reservoir elements which can be acquired accurately. The DOM can also provide the basis of comparative studies on reservoirs in the exploitation of oil and gas.