Reservoir Architectural Analysis of Meandering Channel Sandstone in the Delta Plain Based on the Depositional Process

Key words: 
• reservoir architecture /
• high sinuosity meandering river /
• depositional process /
• Daqing Oilfield

Abstract: Reservoir in Daqing Oilfield, the largest oil field in china, is mainly formed in the huge fluvial-delta system, with hign heterogeneities. Distributary channel and channel sandstone are key sandbodies for hydrocarbon accumulation. After more than 40 years of high speed development, it is now in matural stage with a very high water cut. A good understanding of remaining oil and bypassed oil, which are governed by the baffles and barriers, will be a key work for improving the recovery, while reservoir architectures will give a clue for the barriers and baffles prediction and the oil compartment identification.
  Architecture analysis method, widely used for the outcrop analysis, is a good method to understanding the architecture of fluvial deposit. Although some attempts have been tried for the subsurface reservoir analysis and resulting some progress of this method, there are still many difficulties in the subsurface architecture description. Especially it is hard to give a good explanation for the deposition process of the reservoir for this method focusing on the reorganization and distribution of the elements and not enough attention were paid for the process, which made it a little difficult for geologist to understand the architecture result.
  Here a new method for meandering channel sandstone architecture analysis was provided in the density well spacing area. It is hard to reconstruct the process of the highly cut super-complex sandstone, but it is possible to reconstruct the depositing process of moderately cut sandstone and a good explanation can be made for the relationship of the sandbodies in the channel framework. If the process is correct then the architecture will be more reliable.
  There is four hierarchies of architecture were reconstructed in this method, including composite channel belts, single channel belt, point bar and lateral accretion. The composite channel belt was reconstructed based on the widely spread flooding plain deposit between channel belts in the vertical profile. The flooding deposit maybe eroded by the later channel. The main work is to separate the deposit in vertical dimension and to reconstruct a detailed framefork. Single channel belt will bounded based on the flooding plain deposit between channel belts on the areal dimension. Sometimes the distributary channel had to be separated by the sand distribution.
  Point bar can be recognized on well logging, but it is difficult to make a correlation to get the 3 dimension of it. So it will be reconstructed based on the swing process of the channel. First the final abandoned position for the channel was inferred and abandoned cannels during the swing process were recognized based on the sand thickness and the well logging shape of each well and their areal combination. Then the channel swing direction and part of the channel shape can be guessed. Finally the original place for the channel would be induced based on the swing direction and the sand thickness before the first abandoned channel. After all of these works, the channel belt could be separated to several stage and in each stage there were several point bars.
  Lateral accretion architecture is reconstructed based on distribution of the areal density of accretion shale which can be quantified through its relation to its density and the dip in vertical profile. It is easy to get vertical accretion shale in core or well logging. The dip can be calculated based on the core or very densely spaced well which were drilled on the same well site. 
  Two typical layer formed in the delta plain in North S3 block were select to reconstruct the process of the deposit and the detailed reservoir architecture. There are 38 and 23 point bars were recognized in the two layers each. And the dip of the accretion shale is between 4~10 degree and the areal space among shales is 16~20 meters. Combining these data and the shale distribution in the well, the architecture of the lateral accretion were constructed which gives a good understanding of the detailed architecture of the reservoir channel sandstone in the delta plain.