"[Lapangan “X” merupakan lapangan gas terbesar di delta mahakam dengan luas area permukaan yang mancapai 1350km2 dan total akumulasi gas terproduksi mencapai 8 tcf sejak tahun 1990 hingga saat ini. Penurunan produksi yang cukup tajam melatarbelakangi
pengembangan gas di zona dangkal (shallow gas). Sedimen pada zona dangkal ini tersusun oleh endapan deltaik berumur Miosen Atas – Pliosen dengan batupasir sebagai batuan reservoar utama. Keberadaan fluida gas pada batupasir akan berdampak pada penurunan kecepatan gelombang
P dan densitas batuan sehingga memberikan kontras impendansi akustik yang kuat terhadap
lapisan shale. Kontras impedansi akustik ini terlihat sebagai anomali amplitudo (brightspot)
pada seismik. Adanya kenaikan nilai amplitudo seiring dengan bertambah besarnya sudut
datang menjadi hal yang menarik dalam interpretasi shallow gas ini.
Tujuan dari penelitian ini adalah untuk mendeteksi keberadaan shallow gas di lapangan “X”
menggunakan atribut AVO Sismofacies dengan 2 sumur yang dijadikan referensi untuk
pemodelan synthetic AVO. Penulis menggunakan 2 sumur lainnya sebagai kalibrasi terhadap
anomali AVO dari Sismofacies cube yang dihasilkan.
Metode AVO sismofacies ini tidak menggunakan parameter intercept (A) dan gradient (B)
untuk kalkulasi AVO melainkan menggunakan dua data substack yaitu Near dan Far stack.
Crossplot antara Near dan Far pada zona water bearing sand dan shale diambil untuk
mendapatkan background trend sehingga anomali yang berada diluar trend tersebut dapat
diinterpretasikan sebagai gas sand.
Hasil dari analisis AVO Sismofacies ini cukup baik dan menunjukkan kesesuaian dengan
interpretasi gas di beberapa sumur dan efek Coal berkurang jika dibandingkan Far stack.
Meskipun demikian interpretasi AVO ini sebaiknya diintergrasikan dengan analisis dari
atribut seismik lainnya untuk memperkuat interpretasi;Field “X” is a giant gas field in mahakam delta which cover 1350km2 of the area with total
cummulative gas production has reached 8 tcf since 1990 to recently. A significant
decreasing of gas production has led to produce gas accumulation in shallow zone as an
effort to fight againts this decline. Shallow zone is a deltaic sediments which deposited
during Upper Miocen to Pliocene with dominant reservoir is sandstone.
The presence of gas in sandstone has an impact on decreasing of velocity P as well as density
which giving a contrast of acoustic impedance to the overlaying shale. Contrast of
impedance can be observes in seismic as an amplitude anomaly or so called a brightspot. An
increase of amplitude along the offset become more interesting in shallow gas interpretation.
The aim of this study is to detect shallow gas accumulation di field “X” by using AVO
Sismofacies attribute with 2 wells as references to model respons of AVO. The result of
AVO sismofacies will be a cube and the interpreation will be calibrated with 2 existing wells
containing proven gas bearing sands.
AVO Sismofacies method will introduce Near and Far substack to be used in the calculation
instead of using common AVO paramter intecepth (A) and gradient (B). A crossplot between
substacks will create a background trend from water bearing zone and shale hence any
outliers can, then,be interpreted as gas anomaly.
AVO Sismofacies result is encouraging and some of AVO anomaly has been well calibrated
with existing wells. Coal effect which led to misintepretaion in shallow gas sand is
diminished compared to Far stack. Despite of this result, this anomaly interpretation need to
be intergrated with anothers seismic attribute to gain the level of confidence for shallow gas
interpretation., Field “X” is a giant gas field in mahakam delta which cover 1350km2 of the area with total
cummulative gas production has reached 8 tcf since 1990 to recently. A significant
decreasing of gas production has led to produce gas accumulation in shallow zone as an
effort to fight againts this decline. Shallow zone is a deltaic sediments which deposited
during Upper Miocen to Pliocene with dominant reservoir is sandstone.
The presence of gas in sandstone has an impact on decreasing of velocity P as well as density
which giving a contrast of acoustic impedance to the overlaying shale. Contrast of
impedance can be observes in seismic as an amplitude anomaly or so called a brightspot. An
increase of amplitude along the offset become more interesting in shallow gas interpretation.
The aim of this study is to detect shallow gas accumulation di field “X” by using AVO
Sismofacies attribute with 2 wells as references to model respons of AVO. The result of
AVO sismofacies will be a cube and the interpreation will be calibrated with 2 existing wells
containing proven gas bearing sands.
AVO Sismofacies method will introduce Near and Far substack to be used in the calculation
instead of using common AVO paramter intecepth (A) and gradient (B). A crossplot between
substacks will create a background trend from water bearing zone and shale hence any
outliers can, then,be interpreted as gas anomaly.
AVO Sismofacies result is encouraging and some of AVO anomaly has been well calibrated
with existing wells. Coal effect which led to misintepretaion in shallow gas sand is
diminished compared to Far stack. Despite of this result, this anomaly interpretation need to
be intergrated with anothers seismic attribute to gain the level of confidence for shallow gas
interpretation.]"
