Characteristics of Highstand Delta System and Play in Gentle Slop of Faulted Lacustrine Basin: A Case Study of Jiergalangtu Sag, Erlian Basin

Key words: 
• Erlian Basin /
• Jiergalangtu Sag /
• gentle slope /
• highstand delta system /
• play /
• petroleum accumulation zone

Abstract: In the past more than ten years, distribution and prediction of lowstand sandbodies of LST had been put too much emphasis on during sequence stratigraphy research and petroleum exploration in continental faulted basins. However, the questions about petroleum reservoirs formation of highstand sandbodies of HST were rarely studied. As a result, petroleum exploration of highstand sandbodies of HST was directly restricted. Erlian Basin is located in the northmidland of Inner Mongolia Autonomous Region of China and is composed of many small faulted lacustrine basins of early Crataceous. In general, highstand delta system is well developed and formed the main sandstone reservoir in the gentle slop of faulted lacustrine basins. The petroleum exploration practice shows that highstand delta system that developed widely in gentle slopes, together with specific structural settings, can made many types of play and favorable petroleum accumulation zone. As a result, there is a good prospect that different reservoirs in the gentle slop overlap vertically each other and distribute laterally in a large area. The paper takes an example of Jiergalangtu Sag, which is one of the most representative faulted sags in Erlian Basin, to discuss thoroughly the depositional characteristics and petroleum reservoirs formation conditions of highstand delta system. 
〓〓The Lower Crataceous in the gentle of Jiergalangtu Sag can be divided into five thirdorder sequences that are, respectively, named as SQ1, SQ2, SQ3, SQ4 and SQ5 from bottom to top. SQ1, SQ2, SQ3 and SQ4 are lacustrine sequence and the highstand sandbodies of HST of three sequence such as SQ1, SQ2 and SQ3 are the most oilbearing reservoir. Based on sequence stratigraphy analysis of early Crataceous, according to the result of characteristics of borehole lithology and log, cores observation, sedimentary facies of single well and seismic facies analysis, and so on. two types of coarse clastic highstand delta system that consist of fan delta and braided delta are divided in lacustrine sequence. The fan delta system is mainly distributed in HST of SQ1 and its sedimentary facies belts are imperfect. Its main part is the fan delta front subfacies, while the fan delta plain subfacies is difficult to be preserved because of latter erosion of the top strata of SQ1. The braided delta system is well formed in HST of SQ2 and SQ3. Its sedimentary facies belts, including braided delta plain, braided delta front and prebraided delta subfacies, are completely developed. Two different types of the highstand delta system of above have characteristics of multiphasic and inherited formation. In the end, there are two sandbodies concentrating areas in the neighborhood of Well Ji 45 and Well Ji 36 that located respectively in the north and south of the gentle slop.
〓〓After more than twenty years of exploration, two types and eight kinds of oil pools, including structural reservoir, lithologic and stratigraphic reservoir, have been discovered in the gentle slop. According to the stratigraphic position of reservoirs and the distribution character of different traps in gentle slope, and combing with types of oil pools that have been discovered, three plays that are closely associated with hightstand delta system have been respectively divided. In the vertical direction, they are named as SQ1 play of stratigraphic traps, SQ2 play of lithologic traps and SQ3 play of structural and complex traps. On the basis of distribution character of the plays, petroleum accumulation zones in different domains such as structural reservoir, lithologic and stratigraphic reservoir, etc. are finally predicted. It advances that the inner belt of the gentle slope is favorable petroleum accumulation zone of lithologic and stratigraphic reservoir, while the center and outer belts of the gentle slope are favorable petroleum accumulation zone of structural reservoir. Vertically and laterally, the distribution laws of the plays and petroleum accumulation zones provide scientific reference for evaluation and exploration on the whole. Now, through petroleum exploration that regard the highstand delta systems as the main targets, reserves have been discovered on a large scale and a good effect have been made in the inner, center and outer belts of the gentle slope. This example encourages increased focus on petroleum exploration potential of the highstand sandbodies of HST in faulted lacustrine basins and provides useful reference for petroleum exploration of numerous similar faulted basins in the east of China.