Characteristics of High‐Gamma‐Ray Sandstone Reservoirs for Yanchang Formation in Ordos Basin

Abstract

AbstractOn the basis of geological data and well log interpretation, the characteristics of the high‐gamma‐ray reservoirs for Yanchang Formation are analyzed. The results show that the high‐gamma‐ray reservoirs have the typical logging response characteristics, such as significant spontaneous potential (SP) negative anomalies, high gamma ray, high sonic wave slowness and low resistivity. And these reservoirs also have higher contents of feldspar, thorium (Th), uranium (U) and stable content of potassium (K). The characteristics of Th and U contents are similar to those of the tuffs. The development degree of the high‐gamma‐ray reservoirs increase from Chang 6 oil‐bearing Member to Chang 4+5 oil‐bearing Member, while that of tuffs becomes weaker in this order. The well log curve features of the higher gamma ray, higher acoustic transit time and lower resistivity of the high‐gamma‐ray reservoirs are similar to that of the tuffs. The three characteristics above indicate the sandstone might be produced by deposition of homochronous sedimentary volcano tuff ash or previous tuffs. The research results about the four‐property relationships of the high‐gamma‐ray reservoirs show that: (1) high‐gamma‐ray reservoirs can be identified by the overlap of gamma ray and spontaneous potential curves and overlap of acoustic curve and spontaneous potential curves; (2) high‐gamma‐ray reservoirs have relatively well developed secondary pores with the strong heterogeneity, generally with better physical properties, the differences of amplitude between overlapped acoustic and spontaneous potential curves become greater; (3) the differential amplitude of overlapped calculated spontaneous potential curve and spontaneous potential curve can be used to evaluate the oil‐bearing quality of reservoirs. Bigger amplitude differences of overlapped potential curves imply better oil‐bearing property of the reservoirs.