Sedimentary Characteristics of Shallow Delta Deposits in the Lower Part of Minghuazhen Formation of the BZ 19-4 Oil Fields
BZ 19-4 oil fields is in the south of Bozhong Depression in the Bohai Bay Basin. The lower part of Minghuazhen Formation of this oil fields is deposited when the lacustrine is in shrinking stage. With the study of color, composition, texture, sedimentary structure, lithology profile, characteristics in seismic sections and paleotopography of the deposits, it can be decided that the sedimentary environment of the lower part of Minghuazhen Formation in this area is shallow delta and it is in stage of progradation. In delta plain, the sediments in distributary channel is mainly medium sandstone or fine sandstone which develops cross bedding and horizontal bedding. Erosion surfaces are usually found at the base of the channel deposits. The logging curves of the deposits are mainly in cylinder shape or bell shape. Interdistributary areas of the delta plain is composed of swamps and marshes, where clays and silts predominate. In the delta front where the river meets the lake, the density of the river water carrying sediments is larger than the lake water, so the fresh water flow at the bottom downstream into the lake beyond the river mouth. As the water is shallow, the submarine distributary channels are well developed and extend a long distance which can cut the river mouth bars that have been deposited, so the river mouth bars in this area are not well developed. Just like the distributary channel, the submarine distributary channel mainly deposits medium sandstone or fine sandstone which develops cross bedding and horizontal bedding. At the downstream side of the river mouth bar, sheet sandstone is well developed and the lithology is mainly fine sandstone or very fine sandstone. As the water is shallow, there is no semi-deep or deep lake deposits in the basin and the pro-delta deposits is mainly massive shallow marine mudstone. On the plane, sandstone in this area is widespread and distributed evenly. Vertically, mudstone is interbeded with sandstone, which is mainly river sediments. With high porosity and permeability, the sandstone is a sound oil and gas exploration target.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013